Electrodeposition of Large Area, Angle-Insensitive Multilayered Structural Colors.

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.

Semiconductor Ultramicroelectrodes: Platforms for Studying Charge-Transfer Processes at Semiconductor/Liquid Interfaces.

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)63+ 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.

Reduction of Graphene Oxide Thin Films by Cobaltocene and Decamethylcobaltocene.

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 (∼104 S m-1). 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 sp2 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.