Thermodynamic Design of Electrolyte for CuO/Cu₂O Bilayer by Anodic Electrodeposition

Electrodeposition of multilayered semiconductors requires a bath design to electrodeposit the upper layer(s) without dissolving the base layer(s) below. We present herein a reliable approach to bath design based on thermodynamics from the viewpoint of complexation with ligands. A CuO/Cu₂O bilayer film was targeted as an example. We searched a thermodynamic database of complexation constants for ligands that could form a complex with Cu(II) but not with Cu(I), and identified monoethanolamine as one of the best candidates. Using a Cu(II)-monoethanolamine alkaline aqueous bath, we experimentally confirmed that a CuO upper layer could be deposited without dissolving the Cu₂O base layer. We believe that this design is applicable to other bilayer films produced by electrochemical techniques

Basal-Plane Orientation of Zn Electrodeposits Induced by Loss of Free Water in Concentrated Aqueous Solutions

Concentrated aqueous solutions attract considerable attention because water electrolysis can be suppressed due to a decrease in the amount of free water. The present study focuses on electrodeposition behaviors of metallic zinc (Zn) using concentrated aqueous solutions containing bis(trifluoromethylsulfonyl)amide (Tf₂N⁻) anions. An increase in Tf₂N⁻ concentration significantly enhances water-anion interactions, giving characteristic infrared spectra for the breakdown of the hydrogen-bonding networks of water clusters, i.e. loss of free water. For the Tf₂N⁻ system Zn electrodeposits with the preferred orientation of hcp basal plane was observed, while, for the SO₄²⁻ system with the presence of the hydrogen-bonding networks, preferred orientation of basal plane was not observed. The preferred orientation of basal plane is not attributed to the adsorption of Tf₂N⁻ anions on the electrode, proved by the use of mixed Zn(Tf₂N)₂-ZnSO₄ concentrated solutions. The loss of free water in the concentrated Zn(Tf₂N)₂ solutions will suppress hydrogen adsorption at the cathode to promote surface diffusion of intermediate Zn⁺ adions and growth of Zn crystals. Consequently, the promotions and the easier growth of Zn basal planes with the lowest interfacial free energy will enhance the horizontal growth of Zn basal planes

Ammonium·18-crown-6 bis(trifluoromethylsulfonyl)amide

We report synthesis and characterization of an ammonium-based molten salt, ammonium bis(trifluoromethylsulfonyl)amide-18-crown-6 (1/1), i.e. [NH₄⁺･18C6][Tf₂N⁻] (Tf = SO₂CF₃). Raman spectra shows [NH₄⁺･18C6][Tf₂N⁻] consists of NH₄⁺ ion encapsulated by 18C6 and Tf₂N⁻ anion. The melting point of [NH₄⁺･18C6][Tf₂N⁻] was around 100°C. At 140°C, the viscosity of [NH₄⁺･18C6][Tf₂N⁻] was 14.7 mPa s, the conductivity was 8.0 mS cm⁻¹, and the density was 1.23 g cm⁻³. These properties were comparable to those of common ionic liquids

High-density and low-roughness anodic oxide formed on SiC in highly concentrated LiCl aqueous solution

The wide bandgap and high carrier mobility of silicon carbide (SiC), as well as its physical and chemical stability, make it a promising material for a number of applications. One of the key requirements for these applications involves oxide formation on SiC. The usefulness of the oxide produced by anodizing is, however, limited since the anodic oxide formed on SiC in the usual dilute aqueous solution has a low density and high surface roughness. Here, we consider a new parameter in anodic oxide formation by focusing on the concentration of free water in the electrolyte, using a highly concentrated aqueous solution. In a concentrated solution, oxygen evolution, which results in a reduction in the density of the oxide, is suppressed, and the rate of formation of anodic oxide at defect sites effectively decreases to reduce the surface roughness. Furthermore, an interfacial layer with a higher density than SiO₂ is formed between SiC and SiO₂, buffering the difference in density between them. As a result, we successfully obtained an anodic oxide with a relatively high density and low surface roughness. This study provides a new approach to improving the properties of the anodic oxide formed on SiC

Electrochemically active species in aluminum electrodeposition baths of AlCl3/glyme solutions

Electrochemically active species in aluminum (Al) electrodeposition baths using AlCl3 and less volatile solvents i.e. glymes were investigated. Raman spectroscopy revealed that all the glyme baths contained AlCl₄⁻ anions and Al-Cl-glyme cations as ionic species. Room temperature conductivities were as high as the order of 10⁻³ S cm⁻¹ for the diglyme (G2), triglyme (G3) and tetraglyme (G4) baths, whereas that for the butyl diglyme (butylG2) bath was only 10⁻⁴ S cm⁻¹ due to a lower concentration of ionic species. Surprisingly, electrochemical measurements showed that, among the glyme baths, only the G2 bath enabled electrodeposition of Al. Consequently, despite the similar structures of Al-Cl-glyme complex cations, only the G2 complex cations are electrochemically active. This suggests that the desolvation of glymes from Al-Cl-glyme cations and their subsequent reduction is exceptionally easy for the G2 complexes

An ionic liquid consisting of crown ether-coordinated hydronium cation and amide anion

We first report an ionic liquid (IL) that consists of hydronium (H₃O⁺) ion encapsulated by 18–crown–6–ether (18C6) and an amide anion. The composition of the crown ether–coordinated hydronium amide IL was estimated to be as [H₃O⁺•18C6][Tf₂N⁻]. The presence of H₃O⁺ was revealed from the characteristic bands of the hydronium ion present in the infrared spectra. The melting point of [H₃O⁺•18C6][Tf₂N⁻] was 68–70 °C. At 70 °C, the viscosity of [H₃O⁺•18C6][Tf₂N⁻] was 39.5 mPa s, the conductivity was 1.9 mS cm⁻¹, and the density was about 1.32 g cm⁻³. These bath properties of [H₃O⁺•18C6][Tf₂N⁻] were similar to those of common ILs at room temperature

Electrodeposition of an iron thin film with compact and smooth morphology using an ethereal electrolyte

Electrodeposition of iron (Fe) from an ethereal solution was investigated. The bath consisted of ferrous chloride (FeCl₂), diglyme (G2), and aluminum chloride (AlCl₃), in which iron species were estimated to be [Fe(G2)₂]²⁺ complex cations. The effect of hydrogen gas evolution on the morphology of iron deposits was determined by comparing common aqueous electrolytes. An Fe thin film was fabricated using the FeCl₂–G2–AlCl₃ bath without the influence of hydrogen gas evolution, and the nucleation of Fe was explained by an instantaneous nucleation mechanism. As a result, the surface morphology of the Fe thin film was compact and smooth compared with the cases of aqueous and other nonaqueous electrolytes

Suppression of Fast Proton Conduction by Dilution of a Hydronium Solvate Ionic Liquid: Localization of Ligand Exchange

A dilution effect on the proton conduction of a hydronium solvate ionic liquid [H₃O⁺centerdot18C6]Tf₂N, which consists of hydronium ion (H₃O⁺), 18-crown-6-ether ligand (18C6), and bis[(trifluoromethyl)sulfonyl]amide anion (Tf₂N⁻; Tf = CF₃SO₂), has been studied. When [H₃O⁺･18C6]Tf₂N was diluted using equimolar 18C6 solvent, the distinctive fast proton conduction in [H₃O⁺･18C6]Tf₂N was suppressed in stark contrast to the case of common protic ionic liquids. Nuclear magnetic resonance spectroscopy showed that the fast exchange between free 18C6 molecules and coordinated ones, suggesting that the added solvent had induced a local proton exchange rather than a cooperative proton relay

Proton conduction in hydronium solvate ionic liquids affected by ligand shape

We investigated the ligand dependence of the proton conduction of hydronium solvate ionic liquids (ILs), consisting of a hydronium ion (H₃O⁺), polyether ligands, and a bis[(trifluoromethyl)sulfonyl]amide anion (Tf₂N⁻; Tf = CF₃SO₂). The ligands were changed from previously reported 18-crown-6 (18C6) to other cyclic or acyclic polyethers, namely, dicyclohexano-18-crown-6 (Dh18C6), benzo-18-crown-6 (B18C6) and pentaethylene glycol dimethyl ether (G5). Pulsed-field gradient spin echo nuclear magnetic resonance results revealed that the protons of H₃O⁺ move faster than those of cyclic 18C6-based ligands but as fast as those of acyclic G5 ligands. Based on these results and density functional theory calculations, we propose that the coordination of a cyclic ether ligand to the H₃O⁺ ion is essential for fast proton conduction in hydronium solvate ILs. Our results attract special interest for many electro- and bio-chemical applications such as electrolyte systems for fuel cells and artificial ion channels for biological cells