Facile Synthesis of Germanium Nanoparticles with Size Control: Microwave <i>versus</i> Conventional Heating

Abstract

A facile size-controlled synthesis (microwave/conventional) of quasi-spherical germanium nanoparticles is reported. Oleylamine serves as a solvent, a binding ligand, and a reducing agent in the synthesis. Reactions were carried out with microwave-assisted heating, and the results have been compared with those produced by conventional heating. Germanium iodides (GeI4, GeI2) were used as the Ge precursor, and size control in the range of 4–11 nm was achieved by controlling the ratio of Ge4+/Ge2+ in the precursor mix. Longer reaction times and higher temperatures were also observed to have an effect on the nanoparticle size distribution. Microwave heating resulted in crystalline nanoparticles at lower temperatures than conventional resistive heating because of the ability of germanium iodides to convert electromagnetic radiation directly to heat. The reported approach for germanium nanoparticle preparation avoids the use of strong reducing agents (LiAlH4, n-BuLi, NaBH4) and HF for etching and, thus, can be considered simple, safe, and amenable to industrial-level scaleup. The as-prepared nanoparticles are a stable dispersion (hexane or toluene) for weeks when stored under an inert atmosphere (N2/Ar). The stability of the colloidal dispersion was observed to be dependent on the nanoparticle size, with smaller nanoparticles exhibiting longer stability. On exposure to ambient conditions, oxidation occurs over a period of time and results in slow precipitation of the nanoparticles. The nanoparticles have been characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and spectroscopic techniques (UV-Vis-NIR, FTIR, Raman).