2, Central Michigan University, Mount Pleasant, Michigan, United States
Ferroelectric oxide nanocrystals are of interest for a wide variety of functional applications including data storage, energy storage, and within polymer composites for flexible film capacitors. It is known that in the nanoscale size regime, the ferroelectric properties and structural distortion tend to diminish with decreasing size, though ferroelectric properties and structural distortions have been observed for particles as small as 5 nm in recent years. While the effect of size is fairly well characterized, the influence of shape (e.g., sphere vs cube) and surface termination (surrounding matrix or capping ligands) is not as well understood. With regard to surface environment, ligand exchange methods are often employed to change the surface functionalization of the nanoparticles before they are incorporated in polymer composites. However, the effect of different ligands on the structure is largely unknown. In this work, we employed both neutron total scattering and in situ temperature-dependent X-ray total scattering to explore the local and average structures of barium titanate (BaTiO3) nanocrystals as a function of both size and capping ligand. This study utilizes a series consisting of ~10, ~25, and ~30 nm BaTiO3 nanocubes with either oleic acid (CH3(CH2)7CH=CH(CH2)7COOH) or nitrosonium tetrafluoroborate (NOBF4) capping ligands. The oleic acid ligands attach during hydrothermal synthesis of the nanocubes while the NOBF4 ligands can be attached via a simple solution-phase ligand exchange method. In most prior neutron total scattering studies, the capping ligands contained carbon-hydrogen bonding, the presence of which would obscure the Ti-O pair correlations, a key indication of the ferroelectric distortion. However, the NOBF4-BaTiO3 system uniquely does not contain hydrogen, and has revealed the existence of a local ferroelectric distortion similar to that found in bulk BaTiO3 for all nanocrystal sizes studied. Results will also be presented from a study employing in situ X-ray PDFs with elevated temperature (RT – 150°C), showing the effects of size and capping ligand on the tetragonal-to-cubic phase transition, which occurs in bulk BaTiO3 at 120°C. This work contributes to a better understanding of the interplay between the nanocrystal’s surface environment and structure, and will enable better tuning of ferroelectric properties for composite applications.