Synthesis, Solvent-dependent Self-Assembly and Partial Oxidation of Ultrathin Cerium Fluoride Nanoplatelets

Technical Deep Dive: Synthesis, Solvent-dependent Self-Assembly and Partial Oxidation of Ultrathin Cerium Fluoride Nanoplatelets

Overview

This work investigates the synthesis, characterization, and self-assembly behavior of triangular cerium-based nanoplatelets (NPLs). Key findings:

• An optimized synthesis protocol yields monodisperse triangular NPLs with controlled size and shape.
• Detailed structural analysis reveals the NPLs have a mixed oxyfluoride composition (CeOxFy) rather than the expected pure CeF3.
• The choice of solvent strongly influences the NPLs' self-assembly, leading to diverse structures like dispersed individual particles, columnar stacks, and extended 2D superlattices.
• Solvent properties and pre-existing solution-phase organization are critical factors governing the final self-assembled structures.

Methodology

• Synthesized triangular NPLs via thermal decomposition of cerium trifluoroacetate precursor in oleic acid and octadecene.
• Characterized the NPLs using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and thermogravimetric analysis (TGA).
• Investigated the NPLs' self-assembly behavior by dispersing them in different solvents and observing the structures formed upon solvent evaporation, using small-angle X-ray scattering (SAXS) and TEM.

Results

Structural Characterization

• X-ray diffraction and electron diffraction patterns are more consistent with a CeF3 structure than Ce2O3, but show a small shift in peak positions indicating 1-3% expansive strain.
• XPS and TGA data reveal the NPLs have a mixed oxyfluoride composition (CeOxFy), rather than pure CeF3.
• Oxygen content is estimated to be around 15% based on the TGA data.

Self-Assembly Behavior

• SAXS shows NPLs exhibit different solution-phase organization depending on the solvent:
  • In cyclohexane, the NPLs are individually dispersed.
  • In toluene, the NPLs form face-to-face columnar stacks.
• Upon solvent evaporation:
  • Cyclohexane yields extended 2D hexagonal superlattices with edge-to-edge NPL arrangement.
  • Toluene produces long-range columnar assemblies with NPLs oriented perpendicular to the interface.
• Other solvents (e.g. hexane, THF) result in more disordered, kinetically trapped NPL assemblies.

Interpretation

• The partial oxidation of the expected CeF3 structure to CeOxFy composition may occur during synthesis or upon handling/drying.
• Solvent-mediated interactions and pre-existing solution-phase organization are key factors determining the final self-assembled structures.
• Slow-evaporating solvents like toluene allow NPLs to rearrange into more ordered, thermodynamically favored assemblies.
• Fast-evaporating solvents kinetically trap the NPLs in less ordered configurations.

Limitations & Uncertainties

• The precise CeOxFy composition (x and y values) is not definitively determined.
• The timing and mechanism of the partial oxidation process are not fully clear - it may occur during synthesis, purification, or upon drying/handling.
• The relationship between solution-phase structure and final evaporative assemblies is not quantitatively predicted, only qualitatively correlated.

Future Work

• Systematic investigations to pinpoint the oxidation process and its timing.
• Predictive models to relate solution-phase organization to final evaporative self-assembled structures.
• Exploration of how the mixed oxyfluoride composition impacts the NPLs' functional properties compared to pure CeF3.