Crystallographic Alignment of ZnO Nanorod Arrays on Zn₂GeO₄ Nanocrystals: Promising Lattice-Matched Substrates

We demonstrated that ternary Zn2GeO4 crystals could be used as potential lattice-matched substrates for ZnO nanorod array growth. Single-crystalline Zn2GeO4 nanowires were used as substrates for crystallographic alignment of ZnO nanorod arrays. Structural characterization verified the heteroepitaxial growth between the ZnO c-plane and Zn2GeO4 side facets, which was attributed to the small lattice mismatches. The semiconducting Zn2GeO4 crystals are of potential interest as novel alternative substrates for ZnO nanorod array growth

Solid Polymer Electrolyte with High Ionic Conductivity via Layer-by-Layer Deposition

A novel multilayer system via layer-by-layer (LbL) self-assembly by alternatively packing polyethylene glycol (PEG)−α-cyclodextrin (αCD) complex and poly­(acrylic acid) (PAA) via hydrogen-bonding is designed and investigated in this work. The PEG-αCD inclusion complex is obtained by supramolecular interaction. Multilayer films with different ratios of αCD are fabricated, and the growth behavior, morphology, thermal properties, and electrical properties of the resultant LbL films are systematically characterized. The films with PEG-αCD complex and PAA as building blocks show high ionic conductivity reaching up to 2.5 × 10^–5 S cm^–1 under room temperature (52%RH), which is almost two orders magnitude higher than the PEG/PAA films under the same conditions. The PEG-αCD₁₅/PAA films also possess high water retention due to the large amounts of hydroxyl groups carried by αCD, which enables the films, after being exposed to a high humidity environment, to maintain a high ionic conductivity under a low humidity environment. This PEG-αCD/PAA LbL system provides an insight for designing polymer based solid state electrolyte and its application toward electrochemical devices

Bismuth-Catalyzed Growth of Germanium Nanowires in Vapor Phase

We report the successful synthesis of single crystalline Ge nanowires using Bi as catalyst. To the best of our knowledge, this is the first time Bi was used in vapor phase for Ge nanowire growth. An in situ catalyst evaporation method was used to obtain the high quality Ge nanowires. Diameters of the nanowires are in the range of 10−40 nm and the growth direction is along . Composition analyses showed that the nanowires were composed of Ge while the capping catalyst particles were Bi. Controlled experiments showed that source material with proper Bi/Ge molar ratio was a key aspect for the growth of high purity nanowires. The low-temperature growth of Ge nanowires, enabled by the low eutectic point of Bi/Ge, is especially desired for their potential integration with existing semiconductor technologies

Synthesis and Structure Characterization of Ternary Zn₂GeO₄ Nanowires by Chemical Vapor Transport

Ternary Zn2GeO4 nanowires and their branched structures were successfully synthesized by a chemical vapor transport method. The nanowires showed a rhombohedral crystal phase with dominant growth direction along [110]. The branches exhibited preferential growth directions, which are perpendicular to the backbone due to homoepitaxial growth. The nanowire growth process was explained using vapor−liquid−solid mechanism with Au as catalysts. The present catalytic growth method may offer better control of the morphologies and structures of the Zn2GeO4 nanowires, promoting further study of their properties and applications

Redox Active Polyaniline-h-MoO₃ Hollow Nanorods for Improved Pseudocapacitive Performance

Combinatorial approaches in preparing nanocomposites of transition metal oxides with conductive polymers have gained enormous attention due to their outstanding pseudocapacitive properties which are mainly associated with the solid-state diffusion of electrolyte ions as well as surface or near-surface reversible redox reactions. Here, we elaborate on the interplay of surface-controlled and diffusion-controlled redox reactions based on polyaniline and hexagonal molybdenum trioxide (h-MoO₃) hollow nanorods to realize improved electrochemical performance of the nanocomposite electrode. The cationic species (Ferric ions) were used as the oxidants to polymerize aniline monomers and assist in the formation of h-MoO₃ hollow nanorods. The formation of h-MoO₃ hollow nanorods was realized through the cation exchange-assisted Kirkendall effect driven by ferric ions. The resultant core–shell architecture of the polymerized h-MoO₃ showed improved pseudocapacitive performance (270 F/g) when compared to the pristine h-MoO₃ hollow nanorods (126 F/g) or polyaniline (180 F/g) at a current density of 1 A/g, with enhanced cycling stability

One-Pot Synthesis of Hierarchically Assembled Tungsten Oxide (Hydrates) Nano/Microstructures by a Crystal-Seed-Assisted Hydrothermal Process

Hierarchically assembled tungsten oxide (hydrates) nano/microstructures were synthesized by a crystal-seed-assisted hydrothermal process. Crystal seeds were prepared by refluxing the sol formed by oxidation reaction between W and H2O2. NaCl was used as the capping agent. The assembled nano/microstructures could be controlled to form multiple morphologies: double-layer nano/microbundles, quasi-nanorod arrays, nanorod-spheres, double-layer nano/microdisks and double-layer six-pod prisms. Their morphologies and phase compositions are dependent on crystal seeds, substrate, and precursor concentration. Hierarchical assembly mechanisms are proposed for the formation of the assembled nano/microstructures. The assembled double-layer nano/microstructures are composed of two different phases of tungsten oxide (hydrates), which represent new cases for two-phase nano/microstructures

Kinking-Induced Structural Evolution of Metal Oxide Nanowires into Single-Crystalline Nanorings

We report an innovative method to fabricate single-crystalline nanorings based on the conventional vapor−liquid−solid (VLS) mechanism. The controllable formation of kinks in functional oxide nanowires (NWs) can be employed to fold the VLS-grown NWs into closed ring-shaped nanostructures. Successful syntheses of single-crystalline In2O3 and Zn2GeO4 nanorings were demonstrated. The present work provides an efficient method for nanoring fabrication based on NWs. The functional metal oxide nanomaterials with unique ring-shaped structures are expected to find interesting applications such as wave-guiding and photonic circuits

Stretchable Graphene Thermistor with Tunable Thermal Index

Stretchable graphene thermistors with intrinsic high stretchability were fabricated through a lithographic filtration method. Three-dimensional crumpled graphene was used as the thermal detection channels, and silver nanowires were used as electrodes. Both the detection channel and electrodes were fully embedded in an elastomer matrix to achieve excellent stretchability. Detailed temperature sensing properties were characterized at different strains up to 50%. It is evident that the devices can maintain their functionalities even at high stretched states. The devices demonstrated strain-dependent thermal indices, and the sensitivity of the thermistors can be effectively tuned using strain. The unique tunable thermal index is advantageous over conventional rigid ceramic thermistors for diverse and adaptive applications in wearable electronics

Single Crystalline Semi-Nanotubes of Indium Germanate

Single crystalline In2Ge2O7 nanotubes were synthesized by a thermal evaporation method. The growth mechanism of those novel seminanotubular structures was explained to be a vapor−liquid−solid (VLS) mechanism. Its morphology and structures were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). The semi-nanotubes have widths of 80−500 nm and lengths of 5−20 μm. As-synthesized products were found to be single crystalline In2Ge2O7 with a monoclinic crystal structure. Indium particles serve as catalysts to direct nanotube growth. Germanium concentration nonuniformity is proposed to be the reason for a semi-nanotubular structure nucleation rather than nanotubes or nanowires

Melanin and Polypyrrole-Coated Nanocellulose Hydrogel Networks for Environmental Sensing and Energy Storage

Melanins are black-brown pigments of a specific class of poly indolequinones found in nature and in the human body. They are responsible for photoprotection, radical scavenging, and metal ion chelation. Recently, there has been significant interest in eumelanin as a functional material due to its macromolecular structure and the exploitation of the quinone-hydroquinone redox equilibrium. While eumelanin can be used in many promising applications, it is insoluble in most solvents, limiting its processing into homogeneous materials and coatings. A promising approach is to use a carrier system to stabilize eumelanin by incorporating cellulose nanofibrils (CNFs), a nanoscopic material that originates from plant biomass. In this work, a flexible network consisting of CNFs coupled with vapor-phase polymerized conductive polypyrrole (PPy) is utilized to form a functional eumelanin hydrogel composite (MelaGel) for environmental sensing and battery applications. Flexible sensors for detecting pH or metal ions made from MelaGel can detect both pH values in a range from 4 to 10 and metal ions like zinc(II), copper(II), and iron(III), paving the way for environmental and biomedical sensor applications. The reduced internal resistance in the MelaGel leads to improved charge storage ability compared to synthetic eumelanin composite electrodes. Other noteworthy advantages of the MelaGel are the amphiphilic nature of PPy and the additionally offered redox centers. Lastly, this material was tested in aqueous electrolyte zinc coin cells, where it was shown to have charge/discharge stability for over 1200 cycles, showcasing this MelaGel composite as a promising eumelanin-based composite hybrid sensor/energy storage material