Novel Approaches of Nanoceria with Magnetic, Photoluminescent,
and Gas-Sensing Properties
Leandro S.R. Rocha,* Rafael A.C. Amoresi, Henrique Moreno, Miguel A. Ramirez, Miguel A. Ponce,
Cesar R. Foschini, Elson Longo, and Alexandre Z. Simões

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ABSTRACT: The modification of CeO2 with rare-earth elements opens up a wide range of
applications as biomedical devices using infrared emission as well as magnetic and gas-sensing
devices, once the structural, morphological, photoluminescent, magnetic, electric, and gas-
sensing properties of these systems are strongly correlated to quantum electronic transitions
between rare-earth f-states among defective species. Quantitative phase analysis revealed that
the nanopowders are free from secondary phases and crystallize in the fluorite-type cubic
structure. Magnetic coercive field measurements on the powders indicate that the substitution
of cerium with lanthanum (8 wt %), in a fluorite-type cubic structure, created oxygen vacancies
and led to a decrease in the fraction of Ce species in the 3+ state, resulting in a stronger room-
temperature ferromagnetic response along with high coercivity (160 Oe). In addition to the
magnetic and photoluminescent behavior, a fast response time (5.5 s) was observed after CO
exposure, indicating that the defective structure of ceria-based materials corresponds to the key of success in terms of applications
using photoluminescent, magnetic, or electrical behaviors.

1. INTRODUCTION

The wide theoretical energy gap (Eg = 6.0 eV), high dielectric
constant (ε ≈ 23), and low dielectric loss (tan δ ≈ 0.0064 at 7
GHz)1,2 of Ceria (cerium oxide, CeO2) makes it one of
reactive rare earth oxides. However, experimentally its energy
gap value is close to 3.2 eV because of O (2p) → Ce (4f)
transitions, a mechanism yet to be fully uncovered.3,4 Different
methods have been applied for synthesizing CeO2 crystals,
such as: co-precipitation,5 flow method,6 organometallic
decomposition,7 and sol−gel methods.8 The conventional
hydrothermal and microwave-assisted hydrothermal (MAH)
represent alternatives to these preparation routes, which allow
for the synthesis of pure oxides at low-temperature conditions
(below 200 °C), leading to lower processing times and
associated energy cost.9,10

It is well known that the properties of ceria-based materials
depend on the reduction of Ce4+ to Ce3+ species. Dhara et al.11

showed a blue-shift in the absorption spectrum of CeO2
nanocrystals, owing to the charge transfer mechanism between
O (2p) and Ce (4f) states. Additionally, Abbas et al.12 showed
that the narrowing of the band gap could also be achieved by
increasing the number of oxygen vacancies in the structure by
systematically doping CeO2 nanoparticles, synthesized by the
co-precipitation method Mn. Liu et al.13 reported size-
dependent ferromagnetism (FM) in CeO2 powders synthe-
sized by the precipitation route, with a ferromagnetic
saturation (Ms ≈ 0.08 emu/g) only observed in powders
with average particle size smaller than 20 nm. Similarly, Chen

et al.14 reported room-temperature FM (RT-FM) in CeO2
nanoparticles prepared by the thermal decomposition method
(Ms ≈ 0.12 emu/g). According to literature, oxygen vacancies
at the surface are created because of exchange interactions
between electron spin moments, owing to the high surface-to-
volume ratio, which is associated with the observed FM.15

Recent combined systematic spectroscopy and microscopic
analysis16 demonstrated that differences in electron position
are strongly correlated to the magnetic behavior, with two
types of electron localization revealed, both resulting in the
reduction of the Ce(IV) species. One is located at the Ce atom
adjacent to oxygen vacancies, responsible for the hopping
conduction mechanism, while the other distributes at farther
shell playing the major role in FM. This behavior indicates that
the presence of oxygen vacancies is essential for the formation
of FM in CeO2. In this context, nanocrystalline ceria emerged
because of its particular physicochemical properties, with the
most significant differences between nano and bulk materials
attributed to the extremely high specific surface area because of
size effects and quantum confinement.17−21 The unique
combination of properties of Ceria-based ceramics makes it

Received: December 11, 2019
Accepted: February 28, 2020
Published: June 15, 2020

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https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Leandro+S.R.+Rocha"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdf
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an important multifunctional material with a wide range of
applications, such as catalysts, photocatalysts, solar cells, solid
oxide fuel cells, hydrogen storage materials, oxygen permeation
membranes, optical devices, ultraviolet absorber, polishing
materials, and in the environmental sensing field.22−32 Novel
approaches have been employed for the preparation of ultra-
highly sensitive devices using surface acoustic wave sensors,
which enable detection with the sub-ppb limit.33−35

With the increasing concern regarding the numerous cases
of carbon monoxide (CO) poisoning36 and the increase of
indoor air pollutants, the development of more reliable,
efficient, and low cost gas sensors has become a major driving
force for scientists around the world. Oxide semiconductors
appear to be a suitable choice for gas-sensing devices because
of some unique properties, such as high selectivity, sensitivity,
stability, and reversibility37,38 but also low cost, durability, and
low power consumption. We also must consider other
important parameters such fast response, reduced dimensions,
portability, and real-time detection.39 The microstructure plays
a significant role in improving the sensitivity of sensors.
Composition, surface modification, temperature, and humidity
are also important factors.40,41 Maekawa et al.42 determined
that sensitivity and selectivity to gases depend strongly on the
grain size, and that the presence of larger grains improves
selectivity. Moreover, the parameters of the electron-depletion
regions within the structure of the material help explain the
sensing mechanism. The oxygen present in the atmosphere
adsorbs on the surface and capture electrons from the
conduction band (CB)43,44 to form anionic oxygen species,
depending on the working temperature. These electrons
become trapped on the surface, creating the electron-depleted
region and hence decreasing the conductivity of the sensor.
Finally, the adsorbed oxygen species oxidizes CO to form
carbon dioxide (CO2), leading to an increase in conductivity,45

which represents the response of the sensor.
Besides the technological progresses in gas-sensing materials,

several improvements must be achieved. In previous works, our
group optimized the synthesis of CeO2 nanoparticles and thick
films through the MAH method for pure and rare earth-doped
CeO2,

46 showing the strong influence of lanthanum
modification on the redox (Ce3+/Ce4+) capability. Addition-
ally, we showed that a 8 wt % La-doped CeO2 sample
presented a significant dual sensing behavior when exposed to
CO, with a response-time (tresp) of approximately 5 s and a
reversibly optical reply close to 0.3 s, changing its surface color,
close to 400 °C. This effect is intrinsically related to
ferromagnetic,47 photoluminescent,48 and gas-sensing proper-
ties. Additionally, the electrical conduction of CeO2 films is
virtually dominated by cluster-to-cluster charge transfer
(CCCT) processes,49 indicating an electron quantum tunnel-
ing/hopping mechanism on the film response, which stems
from interaction between intrinsic vacancies among the
clusters, such as [CeO8]O′...[CeO7·VO

••]...[CeO8]O′, which
can still be present even above room temperature.50

Thus, as a natural extension of the previous published
work,46 a combined investigation of pure and lanthanum-
doped CeO2 sphere-like nanostructures and thick films was
performed, highlighting the interplay between the magnetic,
photoluminescent, and gas-sensing behaviors after CO
exposure.

2. RESULTS AND DISCUSSION
2.1. Characterization of the CeO2 Nanoparticles.

2.1.1. X-ray Diffraction and Rietveld Refinement. Figure
1a,b illustrates the X-ray diffraction (XRD) patterns of pure

and lanthanum-doped samples, respectively. Both patterns
exhibit peaks that consistently correspond to the face-centered
cubic fluorite-type structure of cerium oxide, with a space
group Fm3̅m, according to the Joint Committee on Powder
Diffraction Standards (JCPDS) 34-0394 data-set. This
indicates that pure CeO2 was successfully synthesized via this
procedure. By analyzing the diffracted peaks of the La-doped
CeO2 sample, shown in Figure 1b, a widening of the peaks
related to the crystallographic planes (111) and (220) is
observed at 2θ at 28 and 48, respectively, compared to the pure
sample. This is because of the incorporation of La atoms in the
place of Ce, causing symmetry and disorder changes in the
crystallite upon insertion of rare earth.
The XRD patterns were analyzed by Rietveld refinement.

The employed method indicates a good fit between the
calculated and experimentally obtained patterns, which is
supported by the statistical parameters R bragg, Rwp, and Rexp
(Table 1).
GoF values evidence that the refinement is in agreement

with what has been released in the literature, along with the
both experimentally measured lattice parameter (a) and unit
cell volumes, very close to the PDF card (JCPDS 34-0394) for
CeO2 powder in which a ≈ 5.4165 Å.51−53 These values can be
affected by processing temperature and time, heating rate, and
solvents, which directly influences the formation of [CeO8]
rare-earth clusters within the cubic fluorite structure.
Furthermore, they control the occurrence of structural defects,
such as oxygen vacancies. Slightly larger lattice parameter and

Figure 1. Rietveld refinement of pure (a,b) La-doped CeO2
nanopowders.

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volumes are seen for the doped sample in comparison to the
pure sample (Table 1), in accordance to literature.52 In
addition to the refinement, the crystallite size (d) was
calculated using Debye Scherrer’s equation. The calculated
values were of 16.57 and 10.42 nm for the pure and La-doped
CeO2 samples, respectively, which evidences the successful
incorporation of dopant species into the host structure. It is
known that rare earth-doped ceria could create a correspond-
ing number of anion vacancies, resulting in a solid solution
with high ionic conductivity and defective surfaces, which may
enhance its redox properties. Besides, lanthanum prevents the
growth of ceria crystallites in severe oxidizing environments, as
it happened in this case, acting as a stabilizer.54

Additionally, according to the previously referenced CCCT
mechanism as well as to the small polaron theory,55 the
probability of electron tunneling/hopping transitions is
roughly dependent on the distance between two adjacent
sites, and small variations on these certainly influence their
quantum probability of transference among defective clusters
throughout the bulk. Furthermore, these variables can increase
or reduce the occurrence of structural defects, such as oxygen
vacancies, playing a major role in these mechanisms.
2.1.2. Transmission Electron Microscopy. The size and

morphology of the pure and La-doped CeO2 nanoparticles
were analyzed using transmission electron microscopy (TEM),
shown in Figure 2, which reveals that the MAH product
consists of unevenly dispersed quasi-spherical nanostructured
particles with sizes around 10 nm, approximate to the
measured average crystallite sizes (d) of 16.57 and 10.42 nm
for the pure and doped samples, respectively.

Both powder samples presented a formation of agglomer-
ation, owing to the van der Waals forces, which is an indicative
of ultra-fine particles. In order to minimize their surface energy,
the particles have a tendency to form agglomerates, with a
minimum surface-to-volume ratio,56 explaining why the
particles agglomerate, although matching the interplanar
space of the (111) and (200) fringes of a fluorite cubic
structure. The nanocrystallinity and crystallography of the
samples are proven by selected area electron diffraction
analysis (SAED). SAED patterns are shown as insets of Figure
2a,b and exhibits four broad rings with d-spacing, which can be
attributed to (111), (200), (220), and (311) reflections of the
fluorite cubic structure. The ring patterns obtained from the
SAED confirm the high purity, nanocrystallinity, and the
fluorite-type structure of the pure and lanthanum-doped
samples, reassuring the formation of highly crystalline
materials.

2.1.3. Magnetic Properties. Magnetization (M) versus
magnetic field (H) loops were recorded for pure and La-
doped CeO2 samples at 300 K, shown in Figure 3. The
corresponding values of magnetization and coercive fields are
shown in Table 2.

Both samples show weak RT-FM. However, La-doping
causes an increase in both residual (MR) and saturation
magnetization (MS) values (as listed in Table 2), in accordance
with the generation of more ferromagnetic defective species
after the modification with rare-earth elements,57,58 and higher
than the reported Ms for pure CeO2 nanospheres.14,59 In
addition, the magnetic susceptibility (ci) decreases significantly
on the La-doped CeO2 sample in comparison to the pure
sample, which is evidenced by the decrease in the initial slope

Table 1. Rietveld Refinement Parameters for the Pure and Lanthanum-Doped CeO2 Nanopowders

lattice parameters

refined formula (Ce1−(3/4)xLaxO2) a = b = c (Å) α = β = γ unit cell volume (Å3) RB (%) GoF (%) Rwp (%) Rexp (%)

X = 0.00 5.4134 (6) 90 158.64 (6) 3.06 1.05 12.18 9.04
X = 0.08 5.4165 (9) 90 158.91 (8) 4.56 1.05 15.42 11.3

Figure 2. TEM and SAED micrographs of (a) pure and (b) La-doped
CeO2 nanoparticles.

Figure 3. Magnetization vs applied field of pure and La-doped CeO2
nanoparticles.

Table 2. Magnetic Saturation, Reminiscent Magnetization,
and Coercivity

Ce1−(3/4)xLaxO2

sample MS (emu/g) MR (emu/g) Hc (Oe) Φ (nm) d (nm)

X = 0.0 4.8 × 10−3 5.82 × 10−5 0.00 12 16.57
X = 0.08 6.8 × 10−3 3.10 × 10−4 160.00 6 10.42

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on the M−H curve (when H = 0). Sundaresan et al.59 reported
CeO2 nanoparticles with an average size of approximately 15
nm, showing RT-FM with an Ms = 1.9 × 10−3 emu/g, while
ours were 4.8 and 6.8 × 10−3 emu/g. We can also see a strong
increase in coercivity (from 0 to 160 Oe or 0.016 T) for the
L.D.C sample, 8 times higher than Ni-doped CeO2 nano-
particles presented.60 The results obtained here provide
evidence that the La-doped CeO2 sample can indeed raise
the magnetic moment because of the presence of more oxygen
vacancies in the structure, with the consequent generation of a
direct ferromagnetic coupling called F-center exchange
(FCE)61 on neighboring Ce species,62 owing to the presence
of both variable valence states (Ce4+/Ce3+) as well as to
oxygen vacancies. Additionally, the particle size has been
reported to influence the magnetic properties of materials.
Here, the crystallite size is quite small (∼10 nm), leading to
the uncompensated existence of exchanged-interacted spins at
the crystallite surface responsible for the observed ferromag-
netic behavior.63

2.2. Characterization of Thick Films. Because the dual
sensing properties have a high predominance in the CeO2
sample after La modification,46 the correlation between the
short-range structural order, electronic, and sensor effects was
only made for the La-doped CeO2 system before and after
atmosphere exposure.
2.2.1. Raman Spectroscopy. The Raman spectra of the rare

earth-doped film before and after CO exposure (Figure 4)

show the presence of five modes (M1−M5). Near 260 cm−1,
the M1 is associated with a transversely acoustic mode.64 The
M2 mode at 465 cm−1 represents the typical F2g (M2)
symmetric vibrational mode associated to the cubic fluorite
structure of ceria. The low-intensity M3 mode (600 cm−1)
indicates the formation of oxygen vacancies as a result of the
incorporation of La3+ species into the CeO2 structure, while
the vibrational mode M4 indicates the reduction of Ce4+ to
Ce3+.65−67 Close to 960 cm−1, a fifth mode (M5) is detected,
related to the transverse optic mode.68 The purge of CO
resulted in the reduction of M1, M2, and M3 intensities along
with the appearance of M4 mode, which indicates an alteration
of the short-range order symmetry with reduction of Ce4+ to
Ce3+ species, and an increase in the concentration of oxygen-
related defects within the structure of the material,69

corroborating with the magnetic behavior increase.

The symmetric breathing mode of the O atoms around each
cation is linked to the main Raman peak (F2g) near 450 cm−1,
which is nearly independent of the cation mass because only
the O atoms move.70 According to the literature,64 cubic
fluorite structure cerium oxide has Raman-active threefold-
degenerate F2g mode at ∼465 cm−1 shifts, which broadens with
decreasing particle size. For ceria nanoparticles prepared by the
microwave-assisted solvothermal route,71 with a narrow
particle size distribution (2−4 nm), there is a peak shift to
lower frequencies. This effect is a result of lattice expansion
with decreasing particle size. The same shift to lower
frequencies was observed on gadolinium- and lanthanum-
doped ceria.72,73

2.2.2. Gas-Sensing Properties. Figure 5 illustrates the
relative resistance versus temperature curve for the La-doped

CeO2 thick film, measured as a function of Rvac/Rgas, where Rgas
corresponds to the resistance of the films when they reach an
equilibrium state after CO exposure, and Rvac corresponds to
the resistance of films when in vacuum. The sample presented
a similar behavior of n-type semiconductor materials when in
contact with reducing atmospheres,74 responsible for the
injection of electrons toward the bulk along with the
consequent reduction in resistance (RCO). The maximum
response for the system can be seen around 400 °C, with a
relative resistance of ∼4, being close to the working
temperature values reported for carbon monoxide ceria-based
sensors, around 300−500 °C with a 500 ppm detection limit75

as well as to the ceria quantum dot-based films with a working
temperature of 400 °C.76

After determining the working temperature, the resistance
versus time (Figure 6) behavior was studied at 400 °C,
changing the atmosphere from vacuum to synthetic air (∼20%
oxygen) and 50 mmHg of CO (99.99%). Particularly, a
relatively fast response time of 5.5 s can be observed after CO

Figure 4. Raman spectroscopy of the La-doped film before and after
CO exposure.

Figure 5. Relative resistance × temperature (°C) of the La-doped
CeO2 thick film at 50 mmHg of CO(g).

Figure 6. Relative resistance × time (s) for the La-doped CeO2 thick
film under exposure to gaseous atmospheres.

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exposure, slightly higher than the 2 s obtained with a sintered
CeO2 film at 950 °C77 but better than the 9 s reported by
Loṕez-Mena et al.78 with 300 °C-calcined films. The observed
resistance decrease indicates an n-type semiconductor behavior
of the doped material. When the film is exposed to CO, Ce4+

clusters, such as [CeO8]
×, [CeO7·VO

×], and [CeO7·VO
• ], reduce

to Ce3+, represented by [CeO8]O′ clusters, as a consequence of
the transient 4f1 electrons. These electrons migrate between
clusters by the CCCT mechanism (hopping/tunneling), owing
to polarons, with a probability that roughly depends on the

distance between adjacent sites. This charge transfer between
polarons is intrinsically created by the redox process of Ce3+ ↔
Ce4+, according to the exposed atmosphere. Considering the
electrical conduction controlled by the CCCT mechanism,
during CO exposure, in which the oxygen vacancies play the
major role, the energy between the Fermi level and the 4f state
moves in order to generate charge compensation, changing the
number of electrons available for conduction. In previous
works, we realized that the temperature where the CO-sensing
response takes place is greater than 250 °C.46 Additionally, it

Figure 7. PL response of the (a) pure and La-doped CeO2 thick films before (b) and after (c) CO exposure. (d) Depicts the Ce 3d XPS spectra.

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was shown that prior exposure to CO(g) at 400 °C raised the
amount of oxygen vacant sites. This interaction between the
gas and the film surface must be established considering the
reaction with the previous adsorbed oxygen species onto the
CeO2 surface, according to the following equation

+ → + < °− − T2CO O 2CO e ( 100 C)2(ads) 2 (1)

In terms of intrinsic defective clusters and the charge transfer
process among them, the above equation can be represented as

[ · ] ′ + → [ · ] +

< °

• ×

T

CeO V ...O 2CO CeO V 2CO

( 100 C)

7 O 2(ads) 7 O 2

(2)

of which do not influence in the overall sample resistance.
Increasing the working temperature, we have the following
situation

+ → + > °− − TCO O CO e ( 100 C)2 (3)

or, represented by clusters, as

[ · ] ′ + → [ · ] +• ×CeO V ...O CO CeO V CO7 O (ads) 7 O 2 (4)

After oxygen removal from the surface, as a consequence of
the previous reactions, or when samples are exposed to a
vacuum atmosphere, then CO interacts directly with the
clusters

[ ] [ · ]

→ [ ]′ [ · ] +

•

••

CeO ... CeO V ...CO

CeO ... CeO 2V CO

8 7 O (ads)

8 6 O 2 (5)

[ ] [ ] [ · ]

→ [ ]′ [ · ] [ ]′ +

× × ×

•

CeO ... CeO ... CeO V ...CO

CeO ... CeO 2V ... CeO CO

d8 O 8 7 O (ads)

8 6 O 8 2 (6)

[ ] [ ]

→ [ ]′ [ · ] +•

CeO ... CeO ...CO

CeO ... CeO V CO

d8 O 8 (ads)

8 7 O 2 (7)

[ ] [ ] [ ]

→ [ ]′ [ · ] [ ]′ +••

CeO ... CeO ... CeO ...CO

CeO ... CeO V ... CeO CO

d8 O 8 8 O (ads)

8 7 O 8 2 (8)

The influence of temperature on reactions with CO was
previously considered by Lv et al.47 The CCCT mechanism
can also be expressed after CO exposure, according to these
equations, where the electron situated in adsorbed oxygen
species onto the surface returns to the system, reducing the
cluster from [CeO7·VO

• ] to [CeO7·VO
×], with a consequent

decrease of the electrical resistance. We can observe that when
CO reacts at the surface, two electrons become available for
conduction through Ce 4f1 states, thus increasing the number
of Ce3+ species ([CeO8]O′ ) on the right side of the equation,
with the consequent decrease in resistance, as seen in Figure 6.
2.2.3. Photoluminescent Response. Because we have not

reported the photoluminescence (PL) behavior of the pure
sample before, we also have considered this system in this
section.
Therefore, in order to better understand the effect of CO on

the nature of defects along with their influence on the sensing
behavior of the semiconductor thick film when a trivalent
cation (La3+) is introduced in the CeO2 structure, the PL
response of the film was measured. Doping of cerium oxide can
intensify the dynamics of charge transfer and the formation of
permanent dipoles modifying the photoluminescent properties

of this oxide. Figure 7a−c shows the PL emission of the pure
film (a) and La-doped film before (b) and after CO exposure
(c), respectively. Usually, the PL emission for semiconductor
oxides is related to defects in the crystalline structure mainly
with the presence of oxygen vacancies, causing charge transfer
because of the presence of additional energetic levels within
the band gap region.79 Defects are classified according to the
energy levels created between the valence (VB) and CB bands.
Shallow defects are those located near to the edges of VB or
CB, causing emissions in the more energetic region, blue.80

Deep defects are at the energy levels farther away from VB and
CB edges, resulting in emissions in less energetic regions,
red.81 From our results, we can observe a shift toward the blue-
green region (467−544 nm) on the La-doped CeO2 sample in
comparison to the pure sample, which is associated with the
increase of shallow defects for the doped sample. Allied to the
fact that La3+ promotes an increased stress along the c axis of
the crystal structure with photon absorption close to 3 eV, as
observed by UV−vis measurements,46 these results indicate
that the presence of singly ionized oxygen vacancies, VO

• , is
maintained by the disordered state of the clusters in the
structure, being characteristic of shallow defects,82,83 in
accordance with the sensing and magnetic behaviors explained
above. The La-doped CeO2 sample also shows emission in the
infrared (IR) region, 762 nm, which is nonexistent in the
undoped sample. The emission in the IR region is associated to
sites of different symmetries with doping84−86 While doping
ceria with transition metals results in quenched effects on
photoluminescent emission because of the reduction of
defects, doping with La3+ increases the proportion of defects
(Table 3), which results in high influence on the adsorption of
gases on the surface.87,88

Once IR radiation can penetrate human tissues to differing
extents depending on the wavelength range being used, this
feature allows for its application in biomedical devices using
LED arrays for mucositis prevention, wound healing, and tissue
repair.89 Besides being used for rhinitis, arthritis, and jaundice
treatments, LEDs are also used for the relief of stress, seasonal
affective, and biological clock disorders; not to mention their
usage in the field of low-intensity photo-rejuvenation90 as well
as in fluorescence imaging and image-guided surgery.91 A work
reported by Smith et al.92 showed the use of a 780 nm
femtosecond laser as an “optical pacemaker” for heart muscle
cells. Previous studies showed that distinct neural cells can be
stimulated by IR-pulsed radiation.93−95 In a 2002 article by
Schieke et al.,96 human dermal fibroblasts were exposed to IR
radiation in the range of 760−1400 nm, from 10 to 60 min,
showing an upregulation in the Matrix metalloproteinase 1
(MMP-1, the collagenase involved in the normal turnover of
skin collagen). In this way, rare-earth doping opens up a wide
range of possible applications regarding ceria-based nanoma-
terials.

Table 3. XPS Measurements of the Pure and Lanthanum-
Doped CeO2 Sample before and after CO Exposure

sample Odefect Olattice Odefect/Olattice Ce3+

Ce1−(3/4)xLaxO2 area area ratio %
X = 0.00 20 423 91 066 0.22 23.88
X = 0.08 34 045 112 214 0.30 18.58
X = 0.08 (CO) 35 127 96 534 0.36 20.42

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Besides, the addition of carbon monoxide can affect the
formation of superficial defects in which the interaction of
electrons and phonons in the lattice sites may lead to self-
trapping. In such situation, the electrons polarize their
neighboring molecules and become trapped in self-induced
potential wells because of the polarization field generated by
the moving electrons carried through the lattice. The treatment
performed on the film in a reducing atmosphere like CO
provided energy to a great number of electrons that were easily
available for conduction, thereby changing the PL behavior.
When the thick film is exposed to carbon monoxide, Figure 7c,
some electrons, which are excited in the photoluminescent
process absorb energy of photons (hν) and are promoted from
the oxygen 2p states to lanthanum 3d, which results in the
disappearance of the IR emission favoring the formation of
intermediary energy levels within the band gap that no longer
emits radiation in this particular wavelength.97 Something that
must be considered is that the disappearance of the IR
emission with the introduction of CO can be associated to the
decrease of self-trapped charges, less interaction between the
electron and hole, and donor−acceptor recombination.
Therefore, the whole system will be characterized by a
distribution of deep and shallow defects mostly attributed to
singly ionized oxygen vacancies, VO

• , and their defective
complex clusters.98

A weak blue-green emission at 483 nm observed by Phoka et
al.99 was attributed to the excess of surface defects in CeO2.
Palard et al.72 reported a strong UV emission near 400 nm and
a weak blue signal at about 415 nm attributed to surface
defects, as well as two supplementary weak bands probably
attributed to charge transfer from O2− to Ce4+/Ce3+ species.
Wang et al.21 indicated that the emission bands ranging from
400 to 500 nm for CeO2 are attributed to the hopping from
different defect levels of the range from Ce 4f to O 2p band.
Therefore, the luminescence bands ranging from 400 to 550
nm may be attributed to the transition from different defect
levels to the VB. According to Martins and Isolani,100 optical
transitions of Ce 4f to Ce 5d are possible and result in more
intense bands of transitions than Ce 4f to Ce 4f, which are
usually very broad bands.
To better explain the origin of the photoluminescent

contributions in the La-doped CeO2 samples, the following
arguments are proposed according to the X-ray photoemission
spectroscopy (XPS) data evidenced in Table 3, depicted in
Figure 7d. An increase of 8% in the proportion of defective to
lattice-oxygen species as a consequence of La-doping is
observed, likely ascribed to the creation of oxygen vacancies.
This behavior can be depicted as if the lanthanum-doped CeO2
sample presented 30% of vacant oxygen sites in its structure in
the absence of CO, against 36% for the lanthanum-doped
CeO2 after CO exposure (La-doped CeO2CO). Besides, an
increase of almost 2% in the number of Ce3+ species after CO
exposure can be depicted, already suggesting the presence of
defective species, such as Ce(III) species, likely responsible for
the increased green-orange photoluminescent emission.
In this model, the magnitude and structural order−disorder

effects determine the physical properties of the system, likely
influenced by the equilibria of [CeO8] octahedral groups and
their combination with defective neighboring species. Hence,
distortion processes are triggered on rare-earth clusters,
favoring the formation of intermediary energy levels within
the band gap of the material, composed of mixed O 2p states
along with La and Ce f-states (below the CB and slightly above

the Fermi level). During the exposure to CO, some electrons
are promoted from the O 2p states to higher energy levels
through the absorption of energy. This mechanism results in
the formation of self-trapped excitons, that is, trapping of
electrons (e−) by holes (h•), generating polarons with
electronic density that contributes to the magnetic and
electrical behaviors. A photoemission (hν) occurs when
electrons located in higher levels decay to an empty O 2p
state. Thus, the proposed mechanism for photoluminescent
emission is based on the distortion processes of defective
clusters, such as [CeO8]O′ , [CeO7·VO

×], [CeO7·VO
• ], and

[CeO7·VO
••], as well as to their wave−function interactions

and the consequent cluster-to-cluster charge transfer (CCCT),
as a result of CO exposure. Additionally, this behavior can also
be associated with the formation of superficial defects caused
by modifications on the morphology of the thick films101

during synthesis steps and atmosphere exposure.

3. CONCLUSIONS

The photoluminescent, magnetic, and gas-sensing behaviors of
rare earth-doped CeO2 are strongly correlated to the creation
of singly and doubly ionized oxygen vacancy species, which
provokes the conversion of Ce4+ into Ce3+ and vice-versa,
depending on the doping element as well as to the exposed
atmosphere, opening up several possibilities of application
regarding rare earth-modified cerium oxide. Quantitative phase
analysis confirmed that the pure and lanthanum-doped cerium
oxide samples could be well-indexed to a pure cubic structure
of CeO2 (space group: Fm3m), with lattice parameter a =
5.4165 Å. A fraction of the Ce species is in the +3 state, which
explains the weak RT-FM of the sample (Ms ≈ 0.0068 emu/g)
being the ferromagnetic mechanism discussed by FCE. When
the thick film is exposed to the CO atmosphere, some
electrons are promoted from the oxygen 2p states to
lanthanum/cerium f-states. Besides this, La3+ promotes
increased stress along the c axis of the crystal lattice with
photon absorption close to 2.31 eV, indicating that the
disordered states of the clusters are responsible for maintaining
shallow defects, particularly singly ionized oxygen vacancies
(VO

• ). Besides the magnetic and photoluminescent behavior, a
fast response time (5.5 s) was observed after CO exposure,
indicating that the defective structure of ceria-based materials
corresponds to the key of success in terms of photo-
luminescent, magnetic, and electrical applications, owing to
the transient 4f1 electrons that migrate by an activated
tunneling/hopping mechanism of polarons intrinsically created
by the redox processes when doped or exposed to an
atmosphere.

4. EXPERIMENTAL PROCEDURES

4.1. Synthesis and Characterization of the CeO2
Nanoparticles. Ce1−(3/4)xLaxO2 (x = 0.00 and 0.08) powders
were synthesized by a MAH technique. A cerium(IV) nitrate
hexahydrate [Ce(NO3)3·6H2O, 99.9%; Sigma-Aldrich] sol-
ution of 150 mM was prepared under constant stirring for 15
min at room temperature. The experimental procedure for the
preparation of the Ce1−(3/4)xLaxO2 (x = 0.08) was based on
dissolution of the cerium precursor in an aqueous medium
under magnet stirring, and, separately, lanthanum oxide
(La2O3, 99%; Sigma-Aldrich) was dissolved in distilled water
with nitric acid and added to the former solution. The pH of
the resulting mixture was adjusted to 10 using KOH (2 M,

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99.5%, Synth). The resulting solution was transferred into a
sealed Teflon autoclave and placed in a microwave hydro-
thermal oven (2.45 GHz, 800 W). The system was heat treated
at 100 °C for 8 min with a heating rate fixed at 10 °C/min, as
previously published by our group.10 The autoclave was cooled
to room temperature naturally. The powders were centrifuged
and washed with distilled water and then dried at 100 °C in an
oven for 48 h. Powders were characterized using XRD with a
PANalytical X’Pert PRO diffraction system employing Cu Kα
radiation (λ = 0.1542 nm).102 For Rietveld refinement, the
following diffraction parameters were considered: 40 kV, 30
mA, 10° ≤ 2θ ≥ 110°, Δ2θ = 0.02°, λCu Kα monocromatized
by a graphite crystal, divergence slit = 2.0 mm, reception slit =
0.6 mm, and step time = 8 s. The Rietveld refinement method
was calculated using the software GSAS (General Structure
Analysis System)103 and the graphic interface EXPGUI.104

Samples for TEM were obtained by drying droplets of as-
prepared samples from an ethanol dispersion, which was
sonicated for 5 min onto 300 mesh Cu grids. TEM
micrographs and selected area electron diffraction (SAED)
analysis patterns were obtained at an accelerating voltage of
200 kV, on a FEI model Tecnai G2-20 instrument.
Magnetization measurements were done by using a vibrating-
sample magnetometer from Quantum Design.
4.2. Preparation and Characterization of the Thick

Films. To obtain the lanthanum-doped cerium oxide thick
films, a paste was prepared with an organic binder (glycerol)
using the obtained powders. The used solid/organic binder
ratio was 30 mg/0.05 mL. Thick porous films were conformed
using the screen-printing technique onto the insulating
alumina substrate, on which electrodes with an interdigitated
shape had been delineated by sputtering. The insulating
alumina substrates were 96% dense, with a 25 nm thick
titanium layer deposited to improve adhesion, and a 175 nm
thick platinum film deposited over the Ti-layer, both obtained
with RF-sputtering at 5.10−3 bar, using 120 and 80 W. The
interdigitated electrodes were defined in a home-built
micromachining laser (l = 355 nm; f = 2 KHz). The resulting
samples were thermally treated in a dry air atmosphere up to
60 °C for microscopy investigation. Next, the films were
thermally treated up to 380 °C and maintained at this
temperature for 2 h, using a heating rate of 1 °C/min to assure
the elimination of organic compounds. The Raman spectros-
copy characterization of the film before and after CO exposure
was obtained by a LabRAM iHR550 Horiba Jobin Yvon
spectrometer with a spectral resolution of 1 cm−1, with 40
scans in the range of 100−1500 cm−1, coupled to a CCD
detector. The procedure was carried out with an argon-ion
laser, whose wavelength is 514.5 nm and a power of 8 mW.
The photoluminescent spectrum was obtained using thermal
monochromator monospec27 Jarrel-Ash and a photomultiplier
Hamamatsu R446. A laser (Coherent Innova) of wavelength
350.7 nm (2.57 eV), with 200 mW of power was used. All
measurements were carried out at room temperature. The
electrical resistance of the films was measured using an Agilent
3440A multimeter at different temperatures (25−450 °C),
using a heating rate of 2° C/min, with respect to vacuum (10−3

mmHg or 1.33 × 10−6 bar), dry air (∼20% O2 with pressure of
50 mmHg), and CO (50 mmHg) atmospheres. In order to
observe the film reversible behavior under atmosphere
exposure, measurements of resistance versus time were carried
out on the sample steady state, when a constant resistance was
observed. The electric contribution regarding the used

substrate and electrodes was already measured and published
by Schipani et al.,105 showing no significant influences to the
overall response. The local binding structure and surface of the
samples were investigated by X-ray photoemission spectrosco-
py (XPS) measurements, carried out in a Scienta Omicron
ESCA + spectrometer system equipped with a hemispherical
analyzer EA125 and a monochromatic source in Al Kα (hν =
1486.7 eV). The source was used in 280 W, while the
spectrometer worked on a constant energy rate mode of 50 eV.
All data analysis was performed using CASA XPS Software
(Casa Software Ltd, UK). A Shirley background subtraction
was applied with the baseline encompassing the entire
spectrum and correcting the charge effects using the C 1s
peak at 285.0 eV as the charge reference.

■ AUTHOR INFORMATION

Corresponding Author
Leandro S.R. Rocha − Department of Chemistry, Federal
University of Saõ Carlos (UFSCar), Saõ Carlos, Saõ Paulo
13565-905, Brazil; orcid.org/0000-0002-6059-2197;
Email: leandro.rocha@liec.ufscar.br

Authors
Rafael A.C. Amoresi − School of Engineering, Sao Paulo State
University (UNESP), Guaratingueta,́ Saõ Paulo 12516-410,
Brazil; orcid.org/0000-0002-7523-6013

Henrique Moreno − School of Engineering, Sao Paulo State
University (UNESP), Guaratingueta,́ Saõ Paulo 12516-410,
Brazil

Miguel A. Ramirez − School of Engineering, Sao Paulo State
University (UNESP), Guaratingueta,́ Saõ Paulo 12516-410,
Brazil

Miguel A. Ponce − Institute of Materials Science and
Technology Investigation (INTEMA), Mar del Plata 7600,
Argentina

Cesar R. Foschini − School of Engineering, Sao Paulo State
University (UNESP), Bauru, Saõ Paulo 17033-360, Brazil

Elson Longo − Department of Chemistry, Federal University of
Saõ Carlos (UFSCar), Saõ Carlos, Saõ Paulo 13565-905,
Brazil; orcid.org/0000-0001-8062-7791

Alexandre Z. Simo ̃es − School of Engineering, Sao Paulo State
University (UNESP), Guaratingueta,́ Saõ Paulo 12516-410,
Brazil

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.9b04250

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
The authors acknowledge financial support of this research
project by the Brazilian research funding agencies CNPq
573636/2008-7, INCTMN 2008/57872-1, and to FAPESP
grant numbers 2013/07296-2, 2017/19143-7 and 2018/
20590-0.

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