Cathodic electroorganic reaction on silicon oxide dielectric electrode.

The faradaic reaction at the insulator is counterintuitive. For this reason, electroorganic reactions at the dielectric layer have been scarcely investigated despite their interesting aspects and opportunities. In particular, the cathodic reaction at a silicon oxide surface under a negative potential bias remains unexplored. In this study, we utilize defective 200-nm-thick n+-Si/SiO2 as a dielectric electrode for electrolysis in an H-type divided cell to demonstrate the cathodic electroorganic reaction of anthracene and its derivatives. Intriguingly, the oxidized products are generated at the cathode The experiments under various conditions provide consistent evidence supporting that the electrochemically generated hydrogen species, supposedly the hydrogen atom, is responsible for this phenomenon. The electrogenerated hydrogen species at the dielectric layer suggests a synthetic strategy for organic molecules.

Aqueous ionic effect on electrochemical breakdown of Si-dielectric-electrolyte interface.

The breakdown of thin dielectric films (SiO2, Si3N4, HfO2) immersed in aqueous electrolyte was investigated. The current and the kinetics of dielectric breakdown caused by large cathodic electric field applied across the dielectric layer reveal the electrochemical nature of dielectric materials. Electrolytes play a huge role in the established dielectric-electrolyte interface with respect to the overall electrical behavior of the system. Although aqueous cations are considered as spectator ions in most electrochemical systems, in dielectric interfaces the current-potential characteristics depend on the type of cation. Computer simulation based on density functional theory and molecular dynamics showed cations affect the dielectric strength. The responses of various dielectric films to solution components provide invaluable information for dielectric-incorporated electrochemical systems.

Full-Color-Tunable Nanophotonic Device Using Electrochromic Tungsten Trioxide Thin Film.

Color generation based on strategically designed plasmonic nanostructures is a promising approach for display applications with unprecedented high-resolution. However, it is disadvantageous in that the optical response is fixed once the structure is determined. Therefore, obtaining high modulation depth with reversible optical properties while maintaining its fixed nanostructure is a great challenge in nanophotonics. In this work, dynamic color tuning and switching using tungsten trioxide (WO3), a representative electrochromic material, are demonstrated with reflection-type and transmission-type optical devices. Thin WO3 films incorporated in simple stacked configurations undergo dynamic color change by the adjustment of their dielectric constant through the electrochromic principle. A large resonance wavelength shift up to 107 nm under an electrochemical bias of 3.2 V could be achieved by the reflection-type device. For the transmission-type device, on/off switchable color pixels with improved purity are demonstrated of which transmittance is modulated by up to 4.04:1.

A miniaturized solid salt reverse electrodialysis battery: a durable and fully ionic power source.

A novel pump-free miniaturized reverse electrodialysis (RED) system was designed to provide lasting power transduced from salinity gradients, named solid salt RED (ssRED), and this quasi-battery uses a solid salt instead of electrolyte solution for streamlined usage. It is portable, flexible, comparable in size to a universal serial bus flash drive, and easily activated with a small amount of water. It maintains a constant ionic concentration gradient through precipitation reactions between a pair of different salts. This precipitation-assisted solid salt RED (PssRED) is an unprecedented ionic power source as it can generate steady electricity in the absence of a driving pump. The PssRED was successfully coupled with bipolar electrode (BPE) microchip sensors which require stable ionic electricity and a polyelectrolyte ionic diode to realize a fully ionic circuit. It is envisioned that the range of application could be expanded to supply electromotive force to various devices through an ionic charge flow.

Robust and High Spatial Resolution Light Addressable Electrochemistry Using Hematite (α-Fe2O3) Photoanodes.

Light addressable/activated electrochemistry (LAE) has recently attracted attention as it can provide spatially resolved electrochemical information without using pre-patterned electrodes whose sizes and positions are unchangeable. Here, we propose hematite (α-Fe2O3) as the photoanode for LAE, which does not require any sort of surface modification for protection or facilitating charge transfer. As experimentally confirmed with various redox species, hematite is stable enough to be used for repetitive electroanalytical measurements. More importantly, it offers exceptionally high spatial resolution so that the "virtual electrode" is exactly as large as the light spot owing to the short diffusion length of the minority carriers. Quantitative analysis of dopamine in this study shows that the hematite-based photoanode is a promising platform for many potential LAE applications including spatially selective detection of oxidizable biomolecules.

Dielectric Breakdown and Post-Breakdown Dissolution of Si/SiO2 Cathodes in Acidic Aqueous Electrochemical Environment.

Understanding the conducting mechanisms of dielectric materials under various conditions is of increasing importance. Here, we report the dielectric breakdown (DB) and post-breakdown mechanism of Si/SiO2, a widely used semiconductor and dielectric, in an acidic aqueous electrochemical environment. Cathodic breakdown was found to generate conduction spots on the Si/SiO2 surface. Using scanning electrochemical microscopy (SECM), the size and number of conduction spots are confirmed to increase from nanometer to micrometer scale during the application of negative voltage. The morphologies of these conduction spots reveal locally recessed inverted-pyramidal structures with exposed Si{111} sidewalls. The pits generation preceded by DB is considered to occur via cathodic dissolution of Si and exfoliation of SiO2 that are induced by local pH increases due to the hydrogen evolution reaction (HER) at the conduction spots. The HER at the conduction spots is more sluggish due to strongly hydrogen-terminated Si{111} surfaces.

Reverse Electrodialysis-Assisted Solar Water Splitting.

Photoelectrochemical (PEC) water splitting provides an attractive route for large-scale solar energy storage, but issues surrounding the efficiency and the stability of photoelectrode materials impose serious restrictions on its advancement. In order to relax one of the photoelectrode criteria, the band gap, a promising strategy involves complementing the conventional PEC setup with additional power sources. Here we introduce a new concept: solar water splitting combined with reverse electrodialysis (RED). RED is a membrane-based power generation technology that produces an electrochemical potential difference from a salinity gradient. In this study, the RED stack serves not only as a separator, but also as an additional tunable power source to compensate for the limited voltage produced by the photoelectrode. A hybrid system, composed of a single-junction p-Si and a RED stack, successfully enables solar water splitting without the need for an external bias. This system provides flexibility in photoelectrode material selection.