Design, synthesis and characterization of ceria: assessment of crystallite size and intrinsic strain using XRD profile analysis and its photocatalytic applications

Abstract

Ceria (CeO₂) was synthesized via ethylene glycol precursor and then calcined at 500 °C for 2 h (assigned as CeO₂-500) to assess its photocatalytic performance through the degradation of Congo red (CR) and Indigo Carmine (IC) dyes. The physicochemical properties of the catalyst were deeply assessed by using TGA-MS, XRD, SEM–EDX, FTIR, UV–vis-DRS and surface charge (pH _PZC) technique. Here, several methods have been explored to estimate the crystallite size and strain in the CeO₂-500 using X-ray peak profile analysis. XRD results revealed the formation of crystalized face-centered cubic fluorite-type CeO₂ phase with the special group Fm-3 m (N°.225). The results of crystallite size and Microstrain estimated by various methods for the CeO₂-500 sample demonstrate that the Size-Strain Plot (SSP) and Halder–Wagner (H–W) methods are more accurate, the values of R² (R² = 0.9975) are for both models. In comparison with Scherrer, W–H and SSP methods, H–W model exhibited a minimum of microstrain (ε = 0.000225) which indicate the narrower size distribution, trivial strains and the presence of defects and size-shape isotropy in the CeO₂-500 environment. Further, the estimated higher value of strain (ε = 0.109) for SSP method may be accredited to the lattice dislocations. All the methods provide crystallite sizes within 10 nm for the as synthesized CeO₂ NPs, excluding for LSLM model which resulted in a crystallite size of 22.72 nm, which proved to be invalid crystal. The Rietveld refinement was robust and convergence was achieved, yielding to low Rp (3.2808%), Rwp 4.3959%) and χ² (0.6187) difference indices. The crystallite size of 10.8306 nm and microstrain of 0.46588 were obtained. SEM analysis showed spherical-like in shape NPs with strong agglomeration of CeO₂-500. The DRS study estimated band gap energies using both absorption edge wavelengths and the Kubelka–Munk model. The band gap energy of 3.2 eV was obtained for the direct allowed electronic transitions of CeO₂-500 NPs. Functional group, especially the Ce–O bonding, was confirmed by the FTIR data. The pH_PZC of 7.2 was estimated for CeO₂-500. Finally, the synthesized CeO₂-500-based photocatalyst revealed substantially enhanced photocatalytic effectiveness 76.44% of CR and 63.41% of IC removed within 100 min, outperforming all other removal processes. Experimental kinetic study was correlated with the Langmuir–Hinshelwood kinetic model for pseudo-first-order reaction (R² = 0.7903 for CR vs. 0.9705 for IC). The mechanistic understandings for the design of CeO₂-500 NPs photocatalyst and their applications I n the degradation of CR and IC dyes is covered in this investigation. Consequently, the outstanding degradation of CR and IC could be can be synergystically explained by the heterogeneous photocatalysis mechanism through ROS (^·OH and O₂^·−), as the robust oxidizing agents implicated in oxidation and reduction processes, Ce⁴⁺/Ce³⁺ redox system together with copious oxygen vacancies and large intrinsic crystal defects (Ce⁴⁺–O defects sites) as primary driving forces ultimately promote the UVA-light harvesting, facilitate charge separation of carriers by reducing its recombination rate and thus boost their photocayatlytic effectiveness of CeO₂-500 photocatalyst.