Hexagonal and Monoclinic Phases of La₂O₂CO₃ Nanoparticles and Their Phase-Related CO₂ Behavior

In this study, we prepared hexagonal and monoclinic phases of La₂O₂CO₃ nanoparticles by different wet preparation methods and investigated their phase-related CO₂ behavior through field-emission scanning microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, thermogravimetric analysis, CO₂-temperature programmed desorption, and linear sweeping voltammetry of CO₂ electrochemical reduction. The monoclinic La₂O₂CO₃ phase was synthesized by a conventional precipitation method via La(OH)CO₃ when the precipitation time was longer than 12 h. In contrast, the hydrothermal method produced only the hexagonal La₂O₂CO₃ phase, irrespective of the hydrothermal reaction time. The La(OH)₃ phase was determined to be the initial phase in both preparation methods. During the precipitation, the La(OH)₃ phase was transformed into La(OH)CO₃ owing to the continuous supply of CO₂ from air whereas the hydrothermal method of a closed system crystallized only the La(OH)₃ phase. Based on the CO₂-temperature programmed desorption and thermogravimetric analysis, the hexagonal La₂O₂CO₃ nanoparticles (HL-12h) showed a higher surface CO₂ adsorption and thermal stability than those of the monoclinic La₂O₂CO₃ (PL-12h). The crystalline structures of both La₂O₂CO₃ phases predicted by the density functional theory calculation explained the difference in the CO₂ behavior on each phase. Consequently, HL-12h showed a higher current density and a more positive onset potential than PL-12h in CO₂ electrochemical reduction.