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Abstract
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.
Highlights
The synthesis of nanomaterials with controllable morphologies and phases has attracted considerable attention in the fields of materials science and inorganic chemistry because the physicochemical and structural properties of the nanomaterials strongly correlate with the types of crystal structures as well as the morphologies of nanoparticles [1,2,3,4,5,6]
When the precipitation time is increased to 12 and 24 h, the characteristic X-ray diffraction (XRD) peaks in the monoclinic La2O2CO3 phase clearly appear at 2θ = 22.8, 29.3, 31.0, 39.9, and 44.4◦ with a C12/c1 space group (JCPDS 48-1113) (Figure 1(Ab,Ac)), which indicates the prevalence of the hexagonal La2O2CO3 phase during the initial precipitation time, followed by the transformation into the monoclinic La2O2CO3 phase after 12 h of precipitation
The HL samples show the XRD patterns that contain the characteristic peaks in only the hexagonal type II La2O2CO3 phase, regardless of the reaction time during the hydrothermal preparation, which demonstrates that there is no change in the La2O2CO3 phase during the preparation process (Figure 1(Ba–Bc))
Summary
The synthesis of nanomaterials with controllable morphologies and phases has attracted considerable attention in the fields of materials science and inorganic chemistry because the physicochemical and structural properties of the nanomaterials strongly correlate with the types of crystal structures as well as the morphologies of nanoparticles [1,2,3,4,5,6]. The unique properties of nanomaterials can be properly tuned by controlling various factors, which results in potential applications of nanomaterials in catalysis, biological labeling, sensing, and optics [1,7,8,9]. The crystalline structures of La2O2CO3 can be divided into three types of different polymorphs:
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