A nonwoven fabric with paperlike qualities composed of silicon nanowires is reported. The nanowires, made by the supercritical-fluid-liquid-solid process, are crystalline, range in diameter from 10 to 50 nm with an average length of >100 μm, and are coated with a thin chemisorbed polyphenylsilane shell. About 90% of the nanowire fabric volume is void space. Thermal annealing of the nanowire fabric in a reducing environment converts the polyphenylsilane coating to a carbonaceous layer that significantly increases the electrical conductivity of the material. This makes the nanowire fabric useful as a self-supporting, mechanically flexible, high-energy-storage anode material in a lithium ion battery. Anode capacities of more than 800 mA h g(-1) were achieved without the addition of conductive carbon or binder.

Both silicon and germanium are leading candidates to replace the carbon anode of lithium ions batteries. Silicon is attractive because of its high lithium storage capacity while germanium, a superior electronic and ionic conductor, can support much higher charge/discharge rates. Here we investigate the electronic, electrochemical and optical properties of Si(1-x)Gex thin films with x = 0, 0.25, 0.5, 0.75, and 1. Glancing angle deposition provided amorphous films of reproducible nanostructure and porosity. The film's composition and physical properties were investigated by X-ray photoelectron spectroscopy, four-point probe conductivity, Raman, and UV-vis absorption spectroscopy. The films were assembled into coin cells to test their electrochemical properties as a lithium-ion battery anode material. The cells were cycled at various C-rates to determine the upper limits for high rate performance. Adjusting the composition in the Si(1-x)Gex system demonstrates a trade-off between rate capability and specific capacity. We show that high-capacity silicon anodes and high-rate germanium anodes are merely the two extremes; the composition of Si(1-x)Gex alloys provides a new parameter to use in electrode optimization.

We report the synthesis of visible-light-active thin films of p-CuBi2O4 utilizing electrodeposition for the first time. Bimetallic films of Cu and Bi were deposited from a single bath onto fluorine-doped tin oxide (FTO) substrates and thermally oxidized in air. The effects of deposition parameters such as potential, time, and bath concentration were investigated. In general, the films consisted of interconnected particles roughly 250 nm in diameter and showed an onset of light absorption at 680 nm (1.8 eV). The films were characterized by photoelectrochemical (PEC) techniques in order to provide insight into the critical PEC properties of CuBi2O4 such as flat-band potential, photocurrent stability, and incident photon-to-current efficiency (IPCE). Despite the narrow band gap and quite suitable band edge positions for PEC H2 generation, the films’ stable calculated AM1.5G photocurrents in N2 degassed Na2SO4 were relatively small (44–55 μA/cm2 at −0.2 V vs Ag/AgCl) due to maximum UV quantum yields of ∼1–2% that decreased significantly throughout the visible range, becoming negligible near 700 nm. Film stability tests under constant illumination revealed that the films were unstable in acidic electrolyte (pH 3.5) due to the reduction and dissolution of Cu but were stable in basic electrolyte (pH 10.8) after an initial ∼50% decrease in performance.

The mechanical strengths of individual germanium (Ge) nanowires with 111 growth direction and diameters ranging from 23 to 97 nm were measured by bending each with a robotic nanomanipulator in a scanning electron microscope (SEM). The nanowires tolerate diameter-dependent flexural strains of up to 17% prior to fracture, which is more than 2 orders of magnitude higher than bulk Ge. The corresponding bending strength of 18 GPa is in agreement with the ideal strength of 14-20 GPa for a perfect Ge crystal. Nanowires also exhibited plastic deformation at room temperature, becoming amorphous at the point of maximum strain. A bendable, nonwoven fabric, or paper, of Ge nanowires is demonstrated.

Confined nanoscale geometry greatly influences physical transformations in materials. The electron microscope enables direct visualization of these changes. We examined the evolution of a germanium (Ge) nanowire attached to a gold (Au) nanocrystal as it was heated to 900 degrees C. The application of a carbon shell prevented changes in volume and interfacial area during the heating cycle. Au/Ge eutectic formation was visualized, occurring 15 degrees C below the bulk eutectic temperature. Capillary pressure pushed the melt into the cylindrical neck of the nanowire, and Ge crystallized in the spherical tip of the carbon shell. Solid-state diffusion down the length of the confined Ge nanowire was observed at temperatures above 700 degrees C; Au diffusion was several orders of magnitude slower than in a bulk Ge crystal.