The role of ligand acid-base reaction in the facile synthesis of Alkali Metal Neodymium Penta- And Heptafluoride Nanocrystals

Abstract

We report a facile co-precipitation method of synthesizing orthorhombic phase CsNd2F7 and hexagonal phase K2NdF5 nanocrystals, which avoids the use of relatively harsh conditions (e.g., high temperatures and long reaction times in hydrothermal synthesis). By reacting MF (M = Cs and K), Nd(CH3CO2)3, oleic acid (OA), and 1-octadecene in the presence of sufficient amounts of oleylamine ligands (OAm) to ensure a non-acidic environment, CsNd2F7 and K2NdF5 nanocrystals are formed for reaction temperatures (Tr) ranging between 150 and 300 °C. When OAm is in excess of OA and Tr = 300 °C, uniformly distributed CsNd2F7 nanodiscs (∼20 nm in diameter) and sub-10 nm K2NdF5 nanodiscs with an average thickness of 2-3 unit cell layers are formed. When OAm is removed, the acidic mixture produces CsNd2F7 nanocrystals at low Tr (150-200 °C); however, a mixture of NdF3 and Cs-oleate is observed instead when Tr is increased. Similarly, in an acidic enviroment, K2NdF5 nanocrystals rapidly decompose into a mixture of K0.156Nd0.142-yF0.702-3y and y(NdF3) (y < 0.142) at low Tr (150-200 °C), while orthorhombic KNdF4 nanocrystals are synthesized at higher Tr (>200 °C). Therefore, the penta- and heptafluoride nanocrystals are thermodynamically stable only in a basic medium when OA is consumed, in part by its condensation reaction with OAm. We further demonstrate that the CsNd2F7 and K2NdF5 nanocrystals display good light-to-heat conversion efficiencies (16-27%) and computed tomography (CT) attenuation, making them promising theranostic photothermal imaging/therapy and CT contrast agents.