This study investigates the transformation of CeO2 nanostructures through various calcination temperatures and their subsequent impact on morphological, structural, and photocatalytic properties. X-ray diffraction (XRD) analysis reveals the presence of cerium oxycarbonate in the uncalcined samples, transitioning to a face-centered cubic CeO2 phase post-calcination at 500 °C. The scanning electron microscopy (SEM) imaging delineates a morphological evolution from distinct, rod-like structures in the uncalcined state to sintered, agglomerated forms as calcination temperatures ascend from 500 °C to 800 °C. The crystallite size, calculated using Scherrer's Equation, displayed a proportional increase with temperature. The photocatalytic degradation of Congo red dye under UV light was analyzed using UV-Vis spectroscopy, with the calcined samples exhibiting varying degrees of adsorption and photocatalytic activity. The study found that higher calcination temperatures correlate with increased photocatalytic performance, potentially due to enhanced crystallinity. This assertion is supported by pseudo-first-order kinetic modeling, indicating improved photocatalytic efficiency with higher calcination temperatures, underlined by increasing rate constants. These findings underscore the intricate relationship between calcination-induced morphological and structural changes and the photocatalytic prowess of CeO2 nanostructures.
Keywords:
Rare-earth Metal Oxide Adsorption Degradation Wastewater Treatment Cerium Oxide
INTRODUCTION
Photocatalytic degradation has emerged as a compelling method to
tackle the issue of dye pollution in wastewater treatment, due to its
potential for complete mineralization of organic pollutants into
non-toxic byproducts
1,2,3.
Several semiconductors, such as \mathrm{ZnO}ZnO,
\mathrm{TiO_{2}}TiO2,
\mathrm{SnO_{2}}SnO2,
\mathrm{MnO_{2}}MnO2,
\mathrm{Fe_{2}O_{3}}Fe2O3,
\mathrm{NiO}NiO,
\mathrm{WO_{3}}WO3,
\mathrm{Nb_{2}O_{5}}Nb2O5,
and \mathrm{CeO_{2}}CeO2,
have been developed as effective photocatalytic materials
4,5,6,7,8.
Cerium dioxide (\mathrm{CeO_{2}}CeO2),
in particular, has garnered increasing attention in the field of
environmental engineering for its remarkable redox properties, high
oxygen storage capacity, and stability
9,10. These attributes render it an
indispensable material for applications in catalysis, fuel cells, and
as an additive in diesel fuels to reduce emissions
11. Recent advancements in
nanotechnology have further enhanced the utility of
\mathrm{CeO_{2}}CeO2,
especially in nanostructured forms, which exhibit unique properties
attributable to their high surface-to-volume ratio and quantum size
effects 12. These nanostructures are
typically synthesized using various methods, with the hydrothermal
process being one of the most favored due to its relatively mild
conditions, scalability, and the quality of the resultant products
13,14. The
resultant morphologies, including rod-like structures, are often
determined by the synthetic conditions and can be fine-tuned to
optimize the material’s performance for specific applications.
Figure 1. The XRD pattern of (a) calcined
(\mathrm{CeO_{2}}CeO2-500)
vs uncalcined \mathrm{CeO_{2}}CeO2
and (b) \mathrm{CeO_{2}}CeO2
calcined at different temperature. (c) The
\mathrm{CeO_{2}}CeO2
crystallite size calculated from XRD patterns. (d) BET analysis of
the \mathrm{CeO_{2}}CeO2-500
samples.
Our previous study highlighted the potential application of
\mathrm{CeO_{2}}CeO2
nanorods, fabricated using the hydrothermal process, as photocatalysts
in dye degradation 15. However,
their performance needs enhancement to produce high-performance
photocatalytic materials. Literature suggests that increasing the
calcination temperature leads to materials with better photocatalytic
performance 16. The calcination process
plays a pivotal role in tailoring the properties of metal oxide
nanostructures. It is known to induce morphological changes, enhance
crystallinity, and remove organic residues or other volatile
contaminants 17,18. These
alterations are crucial as they directly influence the material’s
catalytic activity, adsorptive capacities, and overall chemical
reactivity. Calcination temperature is a critical parameter; it can
dictate the phase stability and surface characteristics of the
resulting \mathrm{CeO_{2}}CeO2.
While lower temperatures may be sufficient to induce phase changes
from precursors such as cerium oxycarbonate to pure
\mathrm{CeO_{2}}CeO2,
higher temperatures may lead to sintering, growth of crystallite size,
and potentially the formation of secondary phases. This study aims to
understand the relationship between calcination conditions and the
characteristics of the resulting \mathrm{CeO_{2}}CeO2
nanostructures, which is essential for optimizing their performance in
environmental applications.
Figure 2. The SEM images of the (a) uncalcined
\mathrm{CeO_{2}}CeO2,
and calcined \mathrm{CeO_{2}}CeO2
(b) \mathrm{CeO_{2}}CeO2-500,
(d) \mathrm{CeO_{2}}CeO2-600,
(e) \mathrm{CeO_{2}}CeO2-700,
and (f) \mathrm{CeO_{2}}CeO2-800
samples
EXPERIMENTAL SECTION
Materials
Cerium nitrate hexahydrate (\mathrm{Ce(NO_{3})3.6H_{2}O}Ce(NO3)3.6H2O),
urea (\mathrm{CO(NH_{2})2}CO(NH2)2),
and Congo red (CR, C.I. 22120) were purchased from Merck, Darmstadt,
Germany. The deionized (DI) water was used as solvent both during
the hydrothermal and dye degradation measurements. All materials
were used as received without any further purifications.
Preparation of \mathrm{CeO_{2}}CeO2
nanostructures and materials characterizations
The \mathrm{CeO_{2}}CeO2
nanorods were fabricated similar to our previously reported study
15. Typically, 3.0 g of
\mathrm{Ce(NO_{3})3}Ce(NO3)3
and 4.2 g of \mathrm{CO(NH_{2})2}CO(NH2)2
were dissolved in 70 mL deionized water using a magnetic stirrer
until the salt completely dissolved. The mixed solution was then
placed inside a 100-mL Teflon-lined autoclave and sealed tightly for
the hydrothermal process. The hydrothermal growth took place at a
temperature of 100 ^\circ∘C
for 12 hours inside an electric oven. The solid was then washed with
DI water and filtered several times using a vacuum filter. The solid
powder was dried in an electric oven at
80 ^\circ∘C
for 4 hours before calcination. To investigate the effect of
calcination temperature on the dye removal performance, the
calcination temperatures were varied (i.e., 500, 600, 700, and
800 ^\circ∘C)
and the samples were named \mathrm{CeO_{2}}CeO2-500,
\mathrm{CeO_{2}}CeO2-600,
\mathrm{CeO_{2}}CeO2-700,
and \mathrm{CeO_{2}}CeO2-800,
respectively. Each sample was calcined using a predetermined holding
temperature and held for 2 hours, with a ramp of
5 ^\circ∘C/min.
A yellowish powder was obtained for each calcination temperature and
used for materials characterization and dye removal investigations.
To investigate the effect of the calcination temperature on the
\mathrm{CeO_{2}}CeO2
morphology and crystalline structure, scanning electron microscopy
(SEM, JEOL JSM-6510) and X-ray diffractometry (XRD, BRUKER D8
ADVANCE ECO) were used respectively.
Photocatalytic dye degradation measurements
For the photodegradation investigation, we used a photoreactor
chamber equipped with four Phillips UVC lamps (10 W,
\lambda \approx 253.7λ≈253.7
nm). Congo red (CR) aqueous solution was prepared with a
concentration of 10 ppm as the model dye. Typically, 50 mg of
\mathrm{CeO_{2}}CeO2
powder was put into 100 mL CR solutions (10 ppm) and stirred inside
the photoreactor chamber. The reaction was initially taken place
under dark conditions for 30 min to achieve stable adsorption
conditions. After that, the UV lamp was then turned on, and the CR
solution was subjected to UV irradiations. Every 20 min, 3.5 mL
suspension was taken and filtered using polyvinylidene fluoride
(PVDF) syringe filters to separate the
\mathrm{CeO_{2}}CeO2
powder. The absorption spectra of 3.5 mL of each time variation were
then measured by a UV-Vis spectrophotometer (Shimadzu UV-1280). The
measurement was done for all samples (\mathrm{CeO_{2}}CeO2-500,
\mathrm{CeO_{2}}CeO2-600,
\mathrm{CeO_{2}}CeO2-700,
and \mathrm{CeO_{2}}CeO2-800).
The concentration of the CR solutions was determined using the
absorbance value at the maximum wavelength
(\lambda = 498λ=498
nm).
RESULTS AND DISCUSSION
Figure 1 shows the XRD pattern of the uncalcined and
calcined \mathrm{CeO_{2}}CeO2
samples. The uncalcined \mathrm{CeO_{2}}CeO2
samples show characteristics of cerium oxycarbonate
(\mathrm{Ce_{2}(CO_{3})2O.H_{2}O}Ce2(CO3)2O.H2O)
in accordance with the PDF number 44-0617 (Figure 1a), which
is similar to previous literatures 19. This
phase is a well-known oxycarbonate of cerium that is mostly obtained
through precipitation of cerium salt through hydrolysis of urea
20. After calcination
(\mathrm{CeO_{2}}CeO2-500),
a new crystal phase was obtained, which has excellent similarity with
the XRD pattern of face-centered cubic (FCC)
\mathrm{CeO_{2}}CeO2
(PDF#43-1002)
21,22. Previous
studies found that the organic residues obtained during the
hydrothermal process of \mathrm{CeO_{2}}CeO2
start to decompose at a temperature of
320 ^\circ∘C
40,23. Based on that
information, we believed that a calcination temperature of
500 ^\circ∘C
is sufficient to convert cerium oxycarbonate into face-centered cubic
\mathrm{CeO_{2}}CeO224. The peaks were observed at
2\thetaθ
values of 28.5^\circ∘;
33.1^\circ∘;
47.5^\circ∘;
56.3^\circ∘;
59.1^\circ∘;
69.4^\circ∘;
76.7^\circ∘;
79.1^\circ∘;
and 88.4^\circ∘,
which correspond to the (hkl) planes of (111), (200), (220), (311),
(222), (400), (331), (420), and (422), respectively.
Figure 3. The UV-Vis spectra of the Congo red (CR) dyes with
increasing reaction times are shown for (a)
\mathrm{CeO_{2}}CeO2-500,
(b) \mathrm{CeO_{2}}CeO2-600,
(c) \mathrm{CeO_{2}}CeO2-700,
and \mathrm{CeO_{2}}CeO2-800
samples. Panel (e) presents photograph images of the discoloration
of CR solutions after increasing contact time with
\mathrm{CeO_{2}}CeO2-500
and \mathrm{CeO_{2}}CeO2-800.
Panel (f) illustrates the changes in CR peak intensity at
\lambdaλ
of 498 nm. The (g) C/C_0C/C0
and (h) \ln(C_0/C)ln(C0/C)
vs. reaction time are also depicted.
Figure 2 showcases the SEM images of (a,b) uncalcined
\mathrm{CeO_{2}}CeO2,
and calcined \mathrm{CeO_{2}}CeO2
at (c) \mathrm{CeO_{2}}CeO2-500,
(d) \mathrm{CeO_{2}}CeO2-600,
(e) \mathrm{CeO_{2}}CeO2-700,
and (f) \mathrm{CeO_{2}}CeO2-800.
The morphological effects of calcination temperature on
\mathrm{CeO_{2}}CeO2
nanostructures were also investigated using the SEM images featured in
Figure 2
sample without calcination treatment (Figure 2a and
2b) show that the \mathrm{CeO_{2}}CeO2
sample without calcination treatment (Figure [FIGREF:2]a and
[FIGREF:2]b) show that the \mathrm{CeO_{2}}CeO2
has formed into elongated, rod-like structures with a relatively
uniform size and orientation, indicative of the typical morphology of
\mathrm{CeO_{2}}CeO2
fabricated via the hydrothermal process
15-500
(Figure 2c) also shows a relatively uniform rod-like
structure. However, as the calcination temperature were increasing for
32-800
samples in which shown in Figure 2d to 2f a more
distinctive different were observed. Upon calcination, the
\mathrm{CeO_{2}}CeO2
sample after calcinations. For example, the
\mathrm{CeO_{2}}CeO2-500
(Figure [FIGREF:2]c) also shows a relatively uniform rod-like
structure. However, as the calcination temperature were increasing for
\mathrm{CeO_{2}}CeO2-600
to \mathrm{CeO_{2}}CeO2-800
samples in which shown in Figure [FIGREF:2]d to [FIGREF:2]f a more
distinctive different were observed. Upon calcination, the
\mathrm{CeO_{2}}CeO2
samples exhibit a distinct morphological evolution from well-defined,
rod-like structures to increasingly sintered and agglomerated forms.
Initially, the uncalcined \mathrm{CeO_{2}}CeO2
displays discrete and uniform rods, indicative of a lower temperature
synthesis with minimal particle fusion. As the calcination temperature
rises, these rods gradually lose their distinctness; they broaden,
fuse, and exhibit smoother edges - a transformation signifying
increased diffusion and coalescence of particles
33. At the highest temperatures
observed, the rods become almost indistinguishable, with significant
agglomeration leading to a bulkier and denser morphology.
Figures 3a to 3d present the UV-Vis spectroscopy
spectra of Congo red (CR) solutions with varying contact times for all
calcined \mathrm{CeO_{2}}CeO2
samples. The CR solutions exhibit maximum absorption at a wavelength
of 498 nm, with peak intensities decreasing as contact time increases,
indicating a reduction in CR concentration. This trend is consistent
across all samples, demonstrating a clear discoloration of the dye
upon reaction or contact, which is visually confirmed (Figure
3e). Initially red, the CR solutions progressively lighten
upon extended contact with the calcined \mathrm{CeO_{2}}CeO2.
This also agrees with the UV-Vis spectroscopy investigation
previously.
Figure 3f elucidates the concentration changes of the CR
solutions over increasing contact times, assessed by the intensity at
the peak wavelength of 498 nm. In the absence of light, the CR
concentration diminishes to varying extents across the samples, with
decreases of over 44%, 46%, 37%, and 32% after 30 minutes of dark
contact for \mathrm{CeO_{2}}CeO2-500,
\mathrm{CeO_{2}}CeO2-600,
\mathrm{CeO_{2}}CeO2-700,
and \mathrm{CeO_{2}}CeO2-800,
respectively. This behavior is attributed to the adsorptive
capabilities of \mathrm{CeO_{2}}CeO2,
as documented in previous literature
34,35. Previous studies
state that with rising calcination temperatures, which typically
reduce the surface area of the materials, the contribution of
adsorption decreases
36,37.
Under UV irradiation, the CR concentration further declines for all
samples, showcasing their photocatalytic activity. The CR degradation
rate (C/C_0C/C0),
defined as the ratio of the initial CR concentration
(C_0C0)
to the concentration after a certain contact time
(CC),
was calculated to evaluate the photocatalytic behavior of
\mathrm{CeO_{2}}CeO2
at different calcination temperatures (Figure 3f). The
results reveal an accelerated CR concentration reduction for
\mathrm{CeO_{2}}CeO2-800
compared to the other samples. Specifically, after 120 minutes of UV
irradiation, the CR concentration decreased by approximately 39%, 50%,
47%, and 54% for \mathrm{CeO_{2}}CeO2-500,
\mathrm{CeO_{2}}CeO2-600,
\mathrm{CeO_{2}}CeO2-700,
and \mathrm{CeO_{2}}CeO2-800,
respectively. This increase in photocatalytic performance with higher
calcination temperatures is likely due to the enhanced crystallinity
of the materials, as indicated by the XRD results
38,39.
Photocatalytic pseudo-first-order kinetic modeling was employed to
further understand the CR degradation by the calcined
\mathrm{CeO_{2}}CeO2,
based on Equation (1):
where \mathrm{C_{0}}C0,
(CC),
(kk),
and (tt)
are the CR initial concentration (ppm), CR solution concentration at
given time (ppm), the pseudo-first-order constant
(min^-1min−1),
and the contact time (min), respectively. The kinetic modeling,
displayed in Figure 3g, exhibits a linear correlation with
high fitting parameters (R^2R2
values) of 0.96832, 0.98445, 0.97148, and 0.98586 for
\mathrm{CeO_{2}}CeO2-500,
\mathrm{CeO_{2}}CeO2-600,
\mathrm{CeO_{2}}CeO2-700,
and \mathrm{CeO_{2}}CeO2-800,
respectively. Furthermore, the rate constant k shows an increase with
the calcination temperature of the \mathrm{CeO_{2}}CeO2,
implying an enhanced photocatalytic performance.
CONCLUSION
In this study, \mathrm{CeO_{2}}CeO2
nanostructures were successfully fabricated through a hydrothermal
process followed by subsequent calcination at varying temperatures.
SEM imaging revealed that all samples maintained a rod-like structure,
with crystallite sizes increasing alongside the calcination
temperatures. This pattern sharpening and narrowing with elevated
temperatures indicate enhanced crystallization and an increase in the
size of the \mathrm{CeO_{2}}CeO2
particles. The photocatalytic capabilities of the calcined
\mathrm{CeO_{2}}CeO2
were assessed through the degradation of Congo red (CR) dye,
demonstrating promising potential for wastewater treatment
applications. The reduction in CR concentration can be attributed to a
combination of adsorption and photocatalytic degradation mechanisms.
However, it was observed that higher calcination temperatures somewhat
diminish adsorption performance, likely due to alterations in surface
charges and a decrease in material surface area. Conversely, the same
increase in calcination temperatures correlated with improved
photocatalytic activity, which can be ascribed to the higher
crystallinity of the materials. These findings highlight the dual role
of calcination temperature in modulating both the physical
characteristics and functional properties of
\mathrm{CeO_{2}}CeO2
nanostructures. Thus, the optimal calcination temperature for
\mathrm{CeO_{2}}CeO2
nanostructures must balance these effects to maximize their efficacy
in environmental applications, particularly in the domain of pollutant
degradation in water treatment scenarios.
Acknowledgments
This study receives no funding.
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