Electrochemical Liquid-Liquid-Solid Electrodeposition of Ge Nanowire Films with Low Reflectance for Light Management Coatings and High Mass-Loading for High Areal Capacity Li+ Battery Anodes.

A simple method for the electrodeposition of dense semiconductor nanowire films on metal foils via the electrochemical liquid-liquid-solid (ec-LLS) method has been demonstrated. Thin liquid InBiCd coatings on metal foil electrodes were used to seed the Ge nanowire growth in an aqueous electrolyte. The resultant Ge nanowire films were characterized via scanning electron microscopy, X-ray diffraction, angle-resolved reflectance spectra, and cyclic voltammetry both outside and within a coin cell by using a Li counter electrode. High magnification electron microscopy indicated that the nanowire sizes and densities depended on the nature of the underlying metal foil. The highest density and thinnest nanowires were obtained with the Cu foil substrates. The as-deposited Ge nanowires were highly crystalline and exhibited a coiled microstructure. The mass density of the films linearly tracked with a longer electrodeposition time. At the highest mass loadings, the films were visibly dull black and exhibited a wavelength-independent total reflectance of ∼10% across visible and near-infrared wavelengths. Separate specular reflectance measurements confirmed that the total reflectance was dominated by diffuse scattering. Optical simulation corroborated the premise that a high density of randomly oriented Ge nanowires with large-angle orientations significantly reduces the total reflectance of such films. Voltammetric measurements indicated that Li+ insertion took place predominantly in the Ge nanowires, with a high initial areal (gravimetric) discharge capacity of 4.12 mA h cm-2 (1359 mA h g-1). An areal (gravimetric) capacity of 2.10 mA h cm-2 (691 mA h g-1) was retained after 50 cycles while maintaining 98.7% Coulombic efficiency. This work significantly advances the use of ec-LLS as a synthesis approach for functional nanowire film for a high-mass loading anode for Li-ion batteries and low reflectance coatings.