The role of precursor decomposition in the formation of samarium doped ceria nanoparticles via solid-state microwave synthesis

Research Article The role of precursor decomposition in the formation of samarium doped ceria nanoparticles via solid‑state microwave synthesis Avi Bregman1 · Jessica Rimsza2 · Marissa Ringgold1 · Nelson Bell1 · LaRico Treadwell1,2 Received: 17 November 2020 / Accepted: 27 January 2021 © The Author(s) 2021  OPEN Abstract The impact on the final morphology of ceria (CeO2) nanoparticles made from different precursors (commercial: cerium acetate/nitrate) and in house: cerium tri(methylsilyl)amide (Ce-TMSA)) via a microwave solid state reaction has been determined. In all instances, powder X-ray diffraction indicated that the cubic fluorite CeO2 phase (PDF# 04–004-9150, with the space group Fm-3 m) had formed. Scanning electron microscopy (SEM) images revealed spherical nanoparticles were produced from the Ce-TMSA precursor. The commercial acetate and nitrate precursors produced particles with irregular morphology. The roles of the precursor decomposition and binding energy in the synthesis of the nanocrystals with various morphologies, as well as a possible growth mechanism, were evaluated based on experimental and computational data. The formation of spherical shaped nanoparticles was determined to be due to the preferential single-step decomposition of the Ce-TMSA as well as the low activation energy to overcome decomposition. Due to the complicated decomposition of the commercial precursors and high activation energy the resulting particles adopted an irregular morphology. Highly uniform samarium doped ceria (SmxCe1-xO2-δ) nanospheres were also synthesized from Ce-TMSA and samarium tri(methylsilyl)amide (Sm-TMSA). The effects of reaction time and temperature, on the final morphology were observed through SEM. The rapid single-step decomposition of TMSA-based precursors as observed through thermogravimetric analysis (TGA) and confirmed through the calculation of potential energy surfaces and binding energies from density functional theory (DFT) calculations, indicated that nanoparticle formation follows LaMer’s classical nucleation theory. Keywords Solid-state · Microwave · Solvent-free · Doped cerium oxide 1 Introduction Ceria (CeO2) nanomaterials have found widespread use in sensors [1, 2], absorbers, [3] and fuel cells as a solid electrolyte due to their inherent physical properties including, chemical inertness, ionic conductivity, high dielectric constant, and moderate band gap [4, 5]. The size of the Ce4+ cation (0.96 Å) and its inherent crystal structure allows for easy doping of C eO2 by trivalent lanthanide cations (Ln3+); the open structure of the cubic fluorite lattice can accommodate high levels of point defect disorder [6]. This property has led to the development of easily doped CeO2 materials like S mxCe1-xO2-δ and GdxCe1-xO2-δ. Doping CeO2 with Ln3+ cations introduces significant oxygen vacancies which can increase ionic conductivity over neat CeO2. There have been many reports on the synthesis of doped CeO2 materials [7–11], but among these new materials, samarium doped ceria ( SmxCe1-xO2-δ) has emerged as one of the more promising doped C eO2 materials due to the similarities in ionic radii of Ce4+ (0.96 Å) and Sm3+ Supplementary Information The online version of this article (https://doi.org/10.1007/s42452-021-04288-y). * LaRico Treadwell, | 1Sandia National Laboratories, Advanced Materials Lab, Albuquerque, NM 87108, USA. 2Geochemistry Department, Sandia National Laboratories, Albuquerque, NM 87108, USA. SN Applied Sciences (2021) 3:341 | https://doi.org/10.1007/s42452-021-04288-y Vol.:(0123456789) Research Article SN Applied Sciences (2021) 3:341 (1.09 Å); this allows for easy solid solution formation. Upon successful doping, S mxCe1-xO2-δ has reported one of the higher ionic conductivities and particularly low operating temperature requirements [12]. For example, Huang et al. showed a nearly two order of magnitude increase in ionic conductivity at 600 °C by doping C eO2 with 23% samarium [13]. It has been reported that the properties of C eO2-based ceramics are highly dependent on the size, shape, and crystalline phase of the particles [14–16]. For instance, Mai et al. observed an oxygen storage content of 554 μmol O g−1 for C eO2 nanorods compared to an oxygen storage content of 318 μmol O g−1 for CeO2 nanopolyhedra [16]. Ma et al. utilized a novel hydrothermal method to synthesize SmxCe1-xO2-δ nanorods, which due to their increased surface area, displayed a very high power density of 522 mW cm−2 [17]. Since the shape-phase-property relationship is important for various applications, it is critical to establish control over the morphology of CeO2 based materials (i.e., morphology, phase, shape, etc.) as a function of L n3+ doping. Previous efforts to tailor the morphology of various C eO2-based nanoparticles have been investigated using commercially available precursors by altering specific synthesis variables, such as pH [18], precursors [19], and surfactants [20]. Recently, Wang et al. demonstrated the synthesis of CeO2 in the form of stacked nanoplatelets, nanorods, nanosquares, and round nanoplatelets using a solution-based synthesis in the presence of different mineralizers [21]. While morphological control of neat CeO2 and trivalent doped CeO2 has been realized by varying the pH, surfactant, and precursors in solution, to the best of our knowledge there has been no report on the systematic comparison of nonvolatile (commercial/ hydrate/air-stable) vs volatile (in-house/non-hydrate/ air-unstable) precursors for the production of neat and Ln3+-doped CeO2 nanoparticles. A broad variety of synthesis processes has been utilized to make C eO2-based nanomaterials including: hydrothermal methods [22], solvothermal methods [23], co-precipitation [24], sol–gel [23], and microemulsion [25]. While many of these methods have produced high quality materials with good shape and size control, they are reliant on secondary mechanisms/catalysts such as surfactants and pH. Furthermore, they can involve aging steps or complex surfactant washing procedures that make them unattractive for large scale synthesis. As an alternative to solutionbased techniques, solid-state reactions have the potential for direct preparation of economically viable, high-purity, stoichiometric metal oxide nanoparticles while avoiding many of the aforementioned complexities [26]. Solidstate reactions can also be improved upon by utilizing microwave heating. Compared to conventional heating methods, microwave heating is more efficient, has a more Vol:.(1234567890) | https://doi.org/10.1007/s42452-021-04288-y homogenous heating profile, and can lead to higher yields [27]. Given the limited investigations of traditional vs nontraditional precursors to produce faceted nanoparticles, a systematic approach was taken to produce CeO2 and SmxCe1-xO2-δ nanoparticles by decomposing traditional cerium/samarium acetate hydrate (Ln-acetate) and cerium/samarium nitrate hexahydr (...truncated)