1. 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 ZnO, TiO2, SnO2, MnO2, Fe2O3, NiO, WO3, Nb2O5, and CeO2, have been developed as effective photocatalytic materials 4,5,6,7,8. Cerium dioxide (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 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.
Our previous study highlighted the potential application of 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 CeO2. While lower temperatures may be sufficient to induce phase changes from precursors such as cerium oxycarbonate to pure 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 CeO2 nanostructures, which is essential for optimizing their performance in environmental applications.
2. EXPERIMENTAL SECTION
2.1 Materials
Cerium nitrate hexahydrate (Ce(NO3)3.6H2O), urea (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.
2.2 Preparation of CeO2 nanostructures and materials characterizations
The CeO2 nanorods were fabricated similar to our previously reported study 15. Typically, 3.0 g of Ce(NO3)3 and 4.2 g of 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°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°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°C) and the samples were named CeO2-500, CeO2-600, CeO2-700, and CeO2-800, respectively. Each sample was calcined using a predetermined holding temperature and held for 2 hours, with a ramp of 5°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 CeO2 morphology and crystalline structure, scanning electron microscopy (SEM, JEOL JSM-6510) and X-ray diffractometry (XRD, BRUKER D8 ADVANCE ECO) were used respectively.
2.3 Photocatalytic dye degradation measurements
For the photodegradation investigation, we used a photoreactor
chamber equipped with four Phillips UVC lamps (10 W,
3. RESULTS AND DISCUSSION
Figure 1 shows the XRD pattern of the uncalcined and
calcined CeO2 samples. The uncalcined CeO2
samples show characteristics of cerium oxycarbonate
(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 (CeO2-500),
a new crystal phase was obtained, which has excellent similarity with
the XRD pattern of face-centered cubic (FCC) CeO2
(PDF#43-1002)
21,22. Previous
studies found that the organic residues obtained during the
hydrothermal process of CeO2 start to decompose at a
temperature of 320°C
40,23. Based on that
information, we believed that a calcination temperature of 500°C is
sufficient to convert cerium oxycarbonate into face-centered cubic
CeO224. The peaks were
observed at 2
The influence of various calcination temperatures on the
crystalline phase of CeO2 is depicted in Figure
1b. Characteristic peaks of face-centered cubic (FCC)
CeO2 were observed for all samples, without the presence of
additional or secondary cerium phases, confirming the successful
fabrication of CeO2 nanostructures. Furthermore, the peak
intensity and sharpness corresponding to cubic CeO2
increased with the calcination temperature. Figure 1c
presents the crystallite sizes of the CeO2 samples at
different calcination temperatures, calculated from the XRD data using
Scherrer’s Equation 25,26.
The crystallite size was calculated to be 7.2 nm, 8.6 nm, 13.4 nm, and
19.4 nm for the CeO2-500, CeO2-600,
CeO2-700, and CeO2-800, respectively. The
crystallite size increases with increasing calcination temperature.
These findings are consistent with previously reported studies
27,28. We also performed
the nitrogen (N2) adsorption-desorption isotherm using
surface area analyzer (Quantachrome QuadraWin ©2000-16). The quantity
adsorbed during adsorption/desorption process with various relative
pressure were shown in Figure 1d. It shows type IV hysteresis
loop revealing the existence of mesopore similar to previous
literatures
29,30,31
. Moreover, the BET (Brunauer, Emmett and Teller) surface area (SBET)
of the CeO2-500 shows a remarkable value of 105.04
Figure 2 showcases the SEM images of (a,b) uncalcined CeO2, and calcined CeO2 at (c) CeO2-500, (d) CeO2-600, (e) CeO2-700, and (f) CeO2-800. The morphological effects of calcination temperature on CeO2 nanostructures were also investigated using the SEM images featured in Figure 2. Images of the CeO2 sample without calcination treatment (Figure 2a and 2b) show that the CeO2 has formed into elongated, rod-like structures with a relatively uniform size and orientation, indicative of the typical morphology of CeO2 fabricated via the hydrothermal process 15,32. A similar morphology was also observed for the CeO2 sample after calcinations. For example, the CeO2-500 (Figure 2c) also shows a relatively uniform rod-like structure. However, as the calcination temperature were increasing for CeO2-600 to CeO2-800 samples in which shown in Figure 2d to 2f a more distinctive different were observed. Upon calcination, the CeO2 samples exhibit a distinct morphological evolution from well-defined, rod-like structures to increasingly sintered and agglomerated forms. Initially, the uncalcined 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 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 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 CeO2-500, CeO2-600, CeO2-700, and CeO2-800, respectively. This behavior is attributed to the adsorptive capabilities of 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 (
Photocatalytic pseudo-first-order kinetic modeling was employed to further understand the CR degradation by the calcined CeO2, based on Equation (1):
where C0, (
4. CONCLUSION
In this study, 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 CeO2 particles. The photocatalytic capabilities of the calcined 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 CeO2 nanostructures. Thus, the optimal calcination temperature for CeO2 nanostructures must balance these effects to maximize their efficacy in environmental applications, particularly in the domain of pollutant degradation in water treatment scenarios.