Size-Controlled AgI/Ag Heteronanowires in Highly Ordered Alumina Membranes: Superionic Phase Stabilization and Conductivity

Systematic study of ionic conduction in silver iodide/mesoporous alumina composites 2: effects of silver bromide doping

In our preceding paper (Y. Fukui et al., Phys. Chem. Chem. Phys., 2023, 25, 25594-25602), we reported a systematic study of the Ag+-ion conducting behaviour of silver iodide (AgI)-loaded mesoporous aluminas (MPAs) with different pore diameters and AgI-loading ratios. By optimising the control parameters, the Ag+-ion conductivity has reached 7.2 × 10-4 S cm-1 at room temperature, which is more than three orders of magnitude higher than that of bulk AgI. In the present study, the effect of silver bromide (AgBr)-doping in the AgI/MPA composites on Ag+-ion conductivity is systematically investigated for the first time, using variable-temperature powder X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy measurements. The AgBr-doped AgI/MPA composites, AgI-AgBr/MPA, formed a homogeneous β/γ-AgI-structured solid solution (β/γ-AgIss) for the composites with AgBr ≤ 10 mol%, above which the composites underwent a phase separation into β/γ-AgIss and face-centred cubic AgBr solid solutions (AgBrss). The onset temperature of the exothermic peaks attributed to the transition from α-AgI-structured solid-solution phase to β/γ-AgIss or AgBrss decreased with increasing the AgBr-doping ratio. The room-temperature ionic conductivity of the AgI-AgBr/MPA composites exhibited a volcano-type dependence on the AgBr-doping ratio with the highest value (1.6 × 10-3 S cm-1) when the AgBr content was 10 mol%. This value is more than twice as high as that of the highest conducting AgI/MPA found in our previous study.

Systematic study of ionic conduction in silver iodide/mesoporous alumina composites 1: effects of pore size and filling level

A systematic study of Ag+-ion conducting behavior in Ag+-loaded porous materials was conducted over the entire sub-10 nm region for the first time. The effects of the pore diameter of mesoporous aluminas (MPAs) and the amount of silver iodide (AgI) loaded into MPAs were investigated using N2 gas adsorption/desorption, powder X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy measurements. Confinement of AgI in the mesoporous space lowers the phase transition temperature between the β/γ- and α-phases relative to that of bulk AgI. The AgI-loading into the MPAs with smaller pores led to a more significant decrease in the transition temperature, possibly because the smaller AgI nanoparticles in the pores must have a higher surface energy to stabilize the high-temperature phase. The room-temperature ionic conductivity exhibits a volcano-type dependence on the pore diameter with the highest value when AgI was loaded into MPA with a pore diameter of 7.1 nm (7.2 × 10-4 S cm-1 at room temperature). Concerning the 7.1 nm-MPA, the room-temperature ionic conductivity was the highest for the nearly fully occupied composite, which is more than three orders of magnitude higher than that of the bulk AgI. The present study reveals that the Ag+-ion conductivity in AgI/MPA composites can be controlled by optimizing the pore diameter of MPA and the AgI-loading ratio.

Mechanochemistry and the Evolution of Ionic Bonds in Dense Silver Iodide

External mechanical stress alters the nature of chemical bonds and triggers novel reactions, providing interesting synthetic protocols to supplement traditional solvent- or thermo-based chemical approaches. The mechanisms of mechanochemistry have been well studied in organic materials made of a carbon-centered polymeric framework and covalence force field. They convert stress into anisotropic strain which will engineer the length and strength of targeted chemical bonds. Here, we show that by compressing silver iodide in a diamond anvil cell, the external mechanical stress weakens the Ag-I ionic bonds and activate the global diffusion of super-ions. In contrast to conventional mechanochemistry, mechanical stress imposes unbiased influence on the ionicity of chemical bonds in this archetypal inorganic salt. Our combined synchrotron X-ray diffraction experiment and first-principles calculation demonstrate that upon the critical point of ionicity, the strong ionic Ag-I bonds break down, leading to the recovery of elemental solids from a decomposition reaction. Instead of densification, our results reveal the mechanism of an unexpected decomposition reaction through hydrostatic compression and suggest the sophisticated chemistry of simple inorganic compounds under extreme conditions.

Electrokinetic Properties of Mesoporous Vitreous Membranes Doped by Silver-Silver Halides

Silver/silver halide materials are considered as efficient and highly stable plasmonic photocatalysts for the organic pollutant degradation and hydrogen evolution from water splitting under solar irradiation, and they possess promising antibacterial activity. Ordered mesoporous silica materials including porous glasses are considered as the most promising template for silver-containing structures. In the present work, Ag/AgHal-doped (Hal = Cl, Br) vitreous membranes on a base of the mesoporous glasses were prepared via step-by-step single-stage impregnation procedure. The chemical and phase composition of the modified membranes were identified by the X-ray photoelectron spectroscopy, the X-ray diffraction and the energy-dispersive X-ray spectroscopy. The structure and morphology of inner membrane space were studied by the scanning electron microscopy. Electrokinetic properties of the silver-containing vitreous membranes were determined by the differential method and the streaming potential method. The inner membrane space is modified unevenly with appearance of the clearly defined regions with different silver content. The formation of the Ag/AgCl clusters along with the individual nanoparticles over thickness of the 1-mm membrane with mean pore radius of 23 nm was detected. The modification of the pore space by Ag-containing structures and the type of halogen ion almost do not affect the electrochemical behavior of the mesoporous vitreous membranes.

Revealing the truncated conical geometry of nanochannels in anodic aluminium oxide membranes

Anodic aluminium oxide (AAO) membranes with self-ordered nanochannels have become promising candidates for applications in the aspects such as structural coloration, photonic crystals, upconversion luminescence and nanofluidic transport. Also, self-ordered AAO membranes have been extensively used for the fabrication of functional nanostructures such as nanowires, nanotubes, nanoparticles, nanorods and nanopillars. Geometries of nanochannels are crucial for the applications of AAO membranes as well as controlling growth (e.g., nucleation, direction and morphology) and in applications (e.g., optics, magnetics, thermoelectrics, biology, medicine, sensing, and energy conversion and storage) of the functional nanostructures fabricated via AAO template-based methods. However, observation of whole nanochannels with nanometer-resolution in thick AAO membranes remains a fundamental challenge, and the nanochannel geometry has not yet been sufficiently elucidated. Here, for the first time, we use depth-profiling transmission electron microscopy to reveal the truncated conical geometry of whole nanochannels of 70 μm in length. Such shape nonuniformity of the nanochannels leads to different reflectance properties of the different depths of the nanochannels along their long axis for one AAO membrane, which suggests that the nonuniformity result in some effects on applications of the nanostructures. Furthermore, we introduce a shape factor to evaluate the shape nonuniformity and demonstrate that the nonuniformity can be remarkably removed by an effective etching method based on a temperature gradient regime.

Fast Room-Temperature Mg2+ Conductivity in Mg(BH4)2·1.6NH3-Al2O3 Nanocomposites

Design of new functional materials with fast Mg-ion mobility is crucial for the development of competitive solid-state magnesium batteries. Herein, we present new nanocomposites, Mg(BH₄)₂·1.6NH₃-Al₂O₃, reaching a high magnesium conductivity of σ(Mg²⁺) = 2.5 × 10^-5 S cm^-1 at 22 °C assigned to favorable interfaces between amorphous state Mg(BH₄)₂·1.6NH₃; inert and insulating Al₂O₃ nanoparticles; and a minor fraction of crystalline material, mainly Mg(BH₄)₂·2NH₃. Furthermore, quasi-elastic neutron scattering reveals that the Mg²⁺-ion mobility in the solid state appears to be correlated to relatively slow motion of NH₃ molecules rather than the fast dynamics of BH₄^- complexes. The nanocomposite is compatible with a metallic Mg anode and shows stable Mg²⁺ stripping/plating in a symmetric cell and an electrochemical stability of ∼1.2 V. The nanocomposite has high mechanical stability and ductility and is a promising Mg²⁺ electrolyte for future solid-state magnesium batteries.

Size Effect of MgO on the Ionic Conduction Properties of a LiBH4·1/2NH3-MgO Nanocomposite

A solid-state electrolyte (SSE) is the core component for fabricating solid-state batteries competitive with the currently commercial Li-ion batteries. In the present study, a LiBH₄·1/2NH₃-MgO nanocomposite has been developed as a fast Li-ion conductor. The conductive properties depend strongly on the size of MgO nanopowders. By adding MgO nanoparticles, the first-order transition at 55 °C observed in the crystalline LiBH₄·1/2NH₃ is suppressed due to the conversion of LiBH₄·1/2NH₃ into the amorphous state. When the size of MgO decreases from 163.6 to 13.9 nm, the MgO amount required for the phase-transition suppression of LiBH₄·1/2NH₃ decreases linearly from 92 to 75 wt %, accompanied by a significant enhancement of ionic conductivity. The optimized nanocomposite with 75 wt % MgO of size 13.9 nm exhibits a pronouncedly high conductivity of 4.0 × 10^-3 S cm^-1 at room temperature, which is 20 times higher than that of the crystalline LiBH₄·1/2NH₃. Furthermore, a smaller size MgO contributes to a higher electrochemical stability window (ESW) owing to the stronger interfacial interaction via B-O bonds, i.e., an ESW of 4.0 V is achieved with the addition of 75 wt % MgO of size 13.9 nm.

Fabrication and application of nanoporous anodic aluminum oxide: a review

Due to the unique optical and electrochemical properties, large surface area, tunable properties, and high thermal stability, nanoporous anodic aluminum oxide (AAO) has become one of the most popular materials with a large potential to develop emerging applications in numerous areas, including biosensors, desalination, high-risk pollutants detection, capacitors, solar cell devices, photonic crystals, template-assisted fabrication of nanostructures, and so on. This review covers the mechanism of AAO formation, manufacturing technology, the relationship between the properties of AAO and fabrication conditions, and applications of AAO. Properties of AAO, like pore diameter, interpore distance, wall thickness, and anodized aluminum layer thickness, can be fully controlled by fabrication conditions, including electrolyte, applied voltage, anodizing and widening time. Generally speaking, the pore diameter of AAO will affect its specific application to a large extent. Moreover, manufacturing technology like one/two/multi step anodization, nanoimprint lithography anodization, and pulse/cyclic anodization also have a major impact on overall array arrangement. The review aims to provide a perspective overview of the relationship between applications and their corresponding AAO pore sizes, systematically. And the review also focuses on the strategies by which the structures and functions of AAO can be utilized.

Enhanced room temperature ionic conductivity of the LiBH4·1/2NH3–Al2O3 composite

In situ formed amorphous LiBH₄·1/2NH₃ on the surface of Al₂O₃ nanoparticles results in an enhanced ion conductivity of 1.1 × 10⁻³ S cm⁻¹ at room temperature.

Rock-salt and helix structures of silver iodides under ambient conditions

Many different phase structures have been discovered for silver iodides. The β and γ phases were found to be the most common ones at ambient conditions, while the rock-salt phase was found to be stable under pressures between 400 MPa and 11.3 GPa. Recently, the α phase was demonstrated to be stable under ambient conditions when the particle sizes were reduced to below 10 nm. However, no other phase has been reported to be stable for silver iodides under ambient conditions. Rock-salt and helix structures have been found to be stable under ambient conditions in this study. The structures have been characterized by elemental mapping, Raman scattering, and high-resolution transmission electron microscopy. The stabilities of these structures were also confirmed by molecular dynamics and density functional theory.

Synthesis and modelling of the mechanical properties of Ag, Au and Cu nanowires

The recent interest to nanotechnology aims not only at device miniaturisation, but also at understanding the effects of quantised structure in materials of reduced dimensions, which exhibit different properties from their bulk counterparts. In particular, quantised metal nanowires made of silver, gold or copper have attracted much attention owing to their unique intrinsic and extrinsic length-dependent mechanical properties. Here we review the current state of art and developments in these nanowires from synthesis to mechanical properties, which make them leading contenders for next-generation nanoelectromechanical systems. We also present theories of interatomic interaction in metallic nanowires, as well as challenges in their synthesis and simulation.

1D Nanomaterials: Design, Synthesis, and Applications in Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have received extensive attention as ideal candidates for large-scale energy storage systems (ESSs) owing to the rich resources and low cost of sodium (Na). However, the larger size of Na⁺ and the less negative redox potential of Na⁺ /Na result in low energy densities, short cycling life, and the sluggish kinetics of SIBs. Therefore, it is necessary to develop appropriate Na storage electrode materials with the capability to host larger Na⁺ and fast ion diffusion kinetics. 1D materials such as nanofibers, nanotubes, nanorods, and nanowires, are generally considered to be high-capacity and stable electrode materials, due to their uniform structure, orientated electronic and ionic transport, and strong tolerance to stress change. Here, the synthesis of 1D nanomaterials and their applications in SIBs are reviewed. In addition, the prospects of 1D nanomaterials on energy conversion and storage as well as the development and application orientation of SIBs are presented.

Nanoionic devices enabling a multitude of new features

In recent years, interesting nanoscale phenomena caused by physical or chemical effects due to local ion transfers near heterointerfaces using ionic conductors have been discovered, and the so-called "nanoionic devices" (which apply new functionalities gained by exploiting those phenomena) are being proposed. Up until now, nanoionic devices with unique functions that have been unattainable with conventional semiconductor devices have been developed. These devices include brain-type devices, completely solid-state electric-double-layer transistors, multi-functional on-demand-type devices, superconducting elements capable of modulating transition temperature, and so on. As for research and development on nanodevices for next-generation information and communication technologies, expected developments of these devices in the field of solid-state ionics are introduced based on the results of our research.