Silicon Nanoparticles by the Oxidation of [Si4]4-- and [Si9]4--Containing Zintl Phases and Their Corresponding Yield

The Zintl phases with nominal compositions Na4Si4, Rb7NaSi8, and A(12)Si(17) (A = K, Rb, Cs) were utilized as precursors in the synthesis of silicon nanoparticles (Si NPs). The present study characterizes and compares the yields of Si NPs synthesized from Na4Si4, Rb7NaSi8, and A(12)Si(17) (A = K, Rb, Cs). Na4Si4 and Rb7NaSi8 Zintl phases consist of anionic silicon tetrahedra stabilized by group I cations. The A(12)Si(17) (A = K, Rb, Cs) Zintl phases that contain [Si9](4-) and [Si4](4-) clusters have been speculated to be more soluble than the A(4)Si(4) (A = Na, Rb, Cs) Zintl phases that contain solely [Si-4](4-) clusters due to the lower charge density of the [Si-9](4-) cluster. The Zintl phases were reacted with NH4Br in dimethylformamide (DMF) and subsequently capped with allylamine. The Si NPs were characterized by transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), UV-vis, and photoluminescence (PL) spectrophotometry. Furthermore, the Si yields were characterized by inductively coupled plasma mass spectrometry (ICP-MS) to evaluate if the reactions of [Si-9](4-) cluster containing Zintl phases resulted in higher yields of Si NPs. The yield of Si increased with larger or mixed alkali metal Zintl phases, leading to the conclusion that Coulombic interactions between the cations and anions affect the Zintl phases reactivity. The size of the Si NPs also increased with larger and mixed alkali metal cations, resulting in similar NP concentrations regardless of the starting material. With respect to ease of synthesis and yield, Na4Si4 remains the most practical precursor for the solution synthesis of Si NPs; however, the larger and mixed alkali metal precursors show promise for further development.