Template-free electrosynthesis of crystalline germanium nanowires from solid germanium oxide in molten CaCl2–NaCl

• Nano-Ge is prepared by direct electrochemical reduction of solid GeO2 in molten salt.
• High-aspect-ratio, high-purity Ge nanowires (Ge-NWs) were obtained at 600 °C.
• Current efficiency is ~86% and energy consumption is only 3.26 kW h (kg Ge)−1.
• Transmittance of obtained Ge-NWs exceeds 90% between 1000 and 3000 cm−1.

Germanium nano-materials are attractive due to their potential application prospects in electronic, information and energy industry. Herein we report a one-step preparation of crystalline germanium nanowires (Ge-NWs) by direct electrochemical reduction of solid GeO2 in molten CaCl2–NaCl at 600 °C with a current efficiency of ~86% and an energy consumptions as low as 3.26 kW h (kg Ge)−1. The cyclic voltammogram of the GeO2-filled cavity electrode shows that the onset electroreduction potential of GeO2 is −1.0 V vs. Ag/Ag+, much more positive than the reduction potential of cations in the melt. The as-prepared Ge-NWs are of high aspect ratio, high purity and good crystallinity according to the XRD, EDX, SEM and TEM characterizations. The present approach provides a simple and short process for the production of Ge-NWs from a cost-affordable germanium precursor (GeO2). The potential applications of the obtained Ge for photodetector and infrared optical window have been also investigated by Raman and FT-IR measurements.