Microstructural and Morphological Characterization of Magnesium-Assisted Combustion Synthesized CaTiO3 by SEM and HRTEM.

Calcium titanate (CaTiO3) exhibits a perovskite-type structure characterized by high thermal and chemical stability, which confers remarkable physicochemical properties, particularly for heterogeneous photocatalysis applications [1]. Reported microstructural studies indicate that CaTiO3 can display a wide range of morphologies, from irregular polyhedra to well-defined shapes such as cuboctahedra [2]. These studies also report an average particle size ranging from 2.5 µm to 7.5 µm [3], depending on the synthesis method employed. The motivation of this work is to elucidate the morphological characteristics of CaTiO3 obtained via a magnesium-assisted combustion method. In a 1:1 molar ratio, CaCO3 (Desarrollo de Especialidades Químicas, 99% purity) and TiO2 (Spectrum Chemical Mfg. Corp., 99% purity) precursors were mixed for 30 minutes. The resulting powder was subsequently compacted into pellets under a uniaxial pressure of 218 MPa [4]. The pellets were coated with 3 g of metallic magnesium wafers (Superior, 99%) to initiate ignition under atmospheric conditions. The morphological features were evaluated by scanning electron microscopy (SEM, Hitachi SU3500) operated at 30 kV, as well as high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2200FS) operated at 200 kV. X-ray diffraction (XRD) pattern indexing was performed using Match! software [5] and the ICSD reference file No. 01-072-1192. CaTiO3 was confirmed to crystallize in an orthorhombic phase with space group Pcmn (62) and lattice parameters a = 5.367(1) Å, b = 7.643(8) Å, and c = 5.443(9) Å, in agreement with previously reported literature [6]. Figure 1a shows the SEM micrograph, revealing agglomerated CaTiO3 particles with quasi-spherical morphology, attributed to the high synthesis temperature (>1200 °C) [7]. From this micrograph, an average particle size of 197 nm was determined (Figure 1b) by fitting a log-normal distribution function (solid red line) to the particle size histogram obtained using ImageJ software [8]. The Levenberg–Marquardt iterative method was employed for the fitting procedure. Figure 2a presents an HRTEM image of CaTiO3. This micrograph was analyzed using CrysTbox software [9] to obtain the fast Fourier transform (FFT) shown in Figure 2b. With the aid of the corresponding CIF file, the diffraction pattern was indexed. These results are consistent with the XRD analysis, confirming the preferential orientation corresponding to the (121) crystallographic plane family [10]. a) SEM micrograph acquired in secondary electron (SE) mode showing an agglomeration of quasi-spherical CaTiO3 particles. b) Particle size distribution analysis of CaTiO3 prepared by the magnesium-assisted combustion method. a) HRTEM image of CaTiO3. b) Diffraction spot pattern analysis of CaTiO3 via CrysTbox software. c) Crystallographic identification of individual reciprocal lattice vectors and calculation of the corresponding zone axis.