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.
In this work, we focus on understanding the morphology
and photocatalytic
properties of CeO2 nanocrystals (NCs) synthesized via a
microwave-assisted solvothermal method using acetone and ethanol as
solvents. Wulff constructions reveal a complete map
of available morphologies and a theoretical-experimental match with
octahedral nanoparticles obtained through synthesis using ethanol
as solvent. NCs synthesized in acetone show a greater contribution
of emission peaks in the blue region (∼450 nm), which may be
associated with higher Ce3+ concentration, originating
shallow-level defects within the CeO2 lattice while for
the samples synthesized in ethanol a strong orange-red emission (∼595
nm) suggests that oxygen vacancies may originate from deep-level defects
within the optical bandgap region. The superior photocatalytic response
of CeO2 synthesized in acetone compared to that of CeO2 synthesized in ethanol may be associated with an increase
in long-/short-range disorder within the CeO2 structure,
causing the E
gap value to decrease, facilitating
light absorption. Furthermore, surface (100) stabilization in samples
synthesized in ethanol may be related to low photocatalytic activity.
Photocatalytic degradation was facilitated by the generation of ·OH
and ·O2
– radicals as corroborated
by the trapping experiment. The mechanism of enhanced photocatalytic
activity has been proposed suggesting that samples synthesized in
acetone tend to have lower e′h· pair recombination,
which is reflected in their higher photocatalytic response.
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