Reduced graphene oxide (RGO) films have been prepared by immersion of graphene oxide (GO) films at room temperature in nonaqueous solutions containing simple, outer-sphere metallocene reductants. Specifically, solutions of cobaltocene, cobaltocene and trifluoroacetic acid (TFA), and decamethylcobaltocene each showed activity for the rapid reduction of GO films cast on a wide variety of substrates. Each reactant increased the conductivity of the films by several orders of magnitude, with RGO films prepared with either decamethylcobaltocene or cobaltocene and TFA possessing the highest conductivities (∼10 S m). X-ray photoelectron spectroscopy suggested that while all three reagents lowered the content of carbon-oxygen functionalities, solutions of cobaltocene and TFA were the most effective at reducing the material to sp carbon. Separately, Raman spectra and atomic force micrographs indicated that RGO films prepared with decamethylcobaltocene consisted of the largest graphitic domains and lowest macroscopic roughness. Cumulatively, the data suggest that the outer-sphere reductants can affect the conversion to RGO but the reactivity and mechanism depend on the standard potential of the reductant and the availability of protons. This work both demonstrates a new way to prepare high-quality RGO films on a wide range of substrate materials without annealing and motivates future work to elucidate the chemistry of RGO synthesis through the tunability of outer-sphere reductants such as metallocenes.
Factors that affect crystalline growth of germanium (Ge) micro-and nanowires by the electrochemical liquid−liquid solid (ec-LLS) method in water have been identified. Alloys of gallium and indium (Ga 1−x In x ) were used as liquid metal microdroplet electrodes in ec-LLS to determine whether the resultant conductivity of as-grown Ge could be strongly modulated by adjusting the fraction of Ga. Current−potential measurements on individual microwires were performed as a function of alloy composition. All alloys of Ga and In yielded Ge microwires with low resistivity and showed evidence of metal incorporation beyond the solubility limit (i.e., hyperdoping). The observed Ge crystal growth rates were insensitive to the alloy composition. In contrast, lowering the concentration of GeO 2 dissolved in solution and/or increasing the density of droplets noticeably slowed down the microwire growth rate. Cumulatively, these data help define what parameters should be useful in refining the ec-LLS method to produce materials with specific targeted properties.
Semiconductor ultramicroelectrodes (SUMEs) were prepared by photolithographic patterning of defined pinholes in dielectric coatings on semiconductor wafers. Methods are reported for interpreting their electrochemical response characteristics in the absence of illumination. Radial diffusion is reconciled with the diode equation to describe the full voltammetric response, allowing direct determination of heterogeneous charge-transfer rate constants and surface quality. The voltammetric responses of n-type Si SUMEs were assessed and showed prototypical UME characteristics with obtainable current densities higher than those of conventional macroscopic electrodes. The SUME voltammetry proved highly sensitive to both native and intentionally grown oxides, highlighting their ability to precisely track dynamic surface conditions reliably through electrochemical measurement. Subsequently, electron transfer from the conduction band of n-Si SUMEs to aqueous Ru(NH3)6 3+ was determined to occur near optimal exoergicity. In total, this work validates the SUME platform as a new tool to study fundamental charge-transfer properties at semiconductor/liquid junctions.
Crystalline germanium (Ge) micro-and nanowires have been grown from aqueous electrolytes through an electrochemical liquid-liquid-solid (ec-LLS) process using eutectic bismuth indium (e-BiIn) alloy as the liquid metal electrode. This alloy represents the first non-Hg or non-Ga containing liquid metal to be used for the growth of a group IV semiconductor crystal below the boiling point of water. The electrochemical stability of e-BiIn in aqueous electrolyte was assessed through cyclic voltammetry, which showed the metal alloy was destabilized either by anodic oxidation at potentials more positive than −1.0 V vs E(Ag/AgCl) or by cathodic reduction of Bi to BiH 3 at potentials more negative than −1.4 V vs E(Ag/AgCl). Within this potential window, ec-LLS produced Ge micro-and nanowires that were evaluated with scanning electron microscopy, high resolution transmission electron microscopy, and selected area electron diffraction. The cumulative analyses showed pronounced crystallinity in the as-prepared Ge microwires and nanowires, with nanowires showing an unexpected coiled morphology. Atom probe tomography data showed some incorporation of In and Bi at 6 and 3 at. %, respectively. The tomography data further demonstrated that the distribution of the metals was not uniform, as In-rich clusters were observed. A high doping character in the as-prepared Ge nanowires was separately confirmed with two-terminal resistivity measurements. In total, this work not only identifies a new liquid metal type that is amenable for ec-LLS but also suggests strongly that the composition of the liquid metal influences the resultant crystal size, shape, and purity.
We demonstrate structural colors produced by a simple, inexpensive, and nontoxic electrodeposition process. Asymmetric metal–dielectric–metal (MDM) multilayered structures were achieved by sequential electrodeposition of smooth gold, thin cuprous oxide, and finally thin gold on conductive substrates, forming an effective optical cavity with angle-insensitive characteristics. Different colors of high brightness were achieved by simply tuning the thickness of the electrodeposited middle cavity layer. This process is compatible with highly nonplanar substrates of arbitrary shape, size, and roughness. This work is the first demonstration of solution-processed, electrodeposited, MDM film stacks that are uniform over large areas and highlights the clear advantages of this approach over traditional deposition or assembly methods for preparing colored films.
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