Meso-oblate Spheroids of Thermal-Stabile Linker-Free Aggregates with Size-Tunable Subunits for Reversible Lithium Storage

The organization of nanoscale materials as building units into extended structures with specific geometry and functional properties is a challenging endeavor. Hereby, an environmentally benign, simple, and scalable method for preparation of stable, linker-free, self-supported, high-order 3D meso-oblate spheroids of CuO nanoparticle aggregates with size-tunable building nanounits for reversible lithium-ion storage is reported. In contrast to traditional spherical nanoparticle aggregation, a unique oblate spheroid morphology is achieved. The formation mechanism of the unusual oblate spheroid of aggregated nanoparticles is proposed. When tested for reversible lithium ion storage, the unique 3D meso-oblate spheroids of CuO nanoparticle aggregate demonstrated highly improved electrochemical performance (around ∼600 mAh/g over 20 cycles), which could be ascribed to the nanoporous aggregated mesostructure with abundant crystalline imperfection. Furthermore, the size of building units can be controlled (12 and 21 nm were tested) to further improve their electrochemical performance

Synthesis of Au Nanorod@Amine-Modified Silica@Rare-Earth Fluoride Nanodisk Core–Shell–Shell Heteronanostructures

We report here the synthesis of water-dispersible Au nanorod (NR)@ amine-modified silica@rare-earth fluoride (REF₃) nanodisk (ND) heteronanostructures (HNs). The HN fabricated as such represents a rational approach to the concurrent improvements in luminescence, water dispersibility, and other application-specific properties (such as those useful for photothermal therapy). A Au NR is deployed as an internal plasmonic antenna to couple the emission of rare-earth (RE) ions to the surface plasmon resonance (SPR) of the Au NR for enhanced photoluminescence. An intervening amine-modified silica shell is used to attach the ultrathin REF₃, to spatially separate them from the internal Au NR antenna, and to impart good water dispersibility all at the same time. Use of internal Au NR plasmonic antenna rather than the more common approach of external antenna may also be applied to design of other REF₃ HNs to improve the application performance

Prevalence of Anisotropic Shell Growth in Rare Earth Core–Shell Upconversion Nanocrystals

Through a series of carefully executed experiments, we discovered the prevalence of anisotropic shell growth in many upconversion NaREF₄ systems caused by a combination of factors: selective adsorption of ligands on the core surface due to the core crystal structure, ligand etching, and the lattice mismatch between core and shell components. This could lead to incomplete shell formation in core–shell nanocrystals under certain conditions. Shell growth is always faster in the <i>a</i> and <i>b</i> crystallographic directions than in the <i>c</i> direction. In the case of a larger lattice mismatch between the core and shell, shell growth only occurs in the <i>a</i> and <i>b</i> directions resulting in an oblong core–shell structure. These findings are useful for rationalizing shell-dependent emission properties, understanding the emission mechanisms in complex core–shell nanostructures, and for creating accurate models of core–shell designs for multifunctionality and optimal performance in applications

Enhancement Effect of Borate Doping on the Oxygen Evolution Activity of α‑Nickel Hydroxide

The availability of low-cost oxygen evolution catalysts is critical to the successful commercialization of renewable energy conversion and storage systems based on solar energy and water splitting reactions. We demonstrate here that a moderate amount (∼12 wt %) of borate doping of crystalline α-Ni­(OH)₂, a common low-cost catalyst, can lower the Tafel slope of the oxygen evolution reaction (OER) in 0.1 M KOH aqueous solution from the typical value of 60 to 43 mV decade^–1 and the overpotential for 10 mA cm^–2 of current density from 390 to 340 mV. Borate doping also improves the stability of α-Ni­(OH)₂ in OER. The borate-doped α-Ni­(OH)₂ can even surpass the benchmark Ir/C precious metal catalyst in terms of activity and stability. These results demonstrate the potential of borate doping as a strategy to increase the catalytic activity of base metal water oxidation catalysts

A Polymer-Infused Solid-State Synthesis of a Long Cycle-Life Na₃V₂(PO₄)₃/C Composite

The cost of large-scale battery-based energy storage systems can be substantially reduced by using long cycle-life active battery materials which can be easily synthesized. This paper describes the synthesis of a sodium vanadium phosphate (Na₃V₂(PO₄)_3, NVP)/carbon composite as the sodium ion battery (NIB) cathode material by a facile solid-state method which can increase the cost competitiveness of NIBs relative to the lithium ion batteries (LIBs). The NVP synthesized as such features NVP particles tethering to a cellular carbon network formed <i>in situ</i> by the carbonization of a low melting polymer percolating the NVP particles. The immobilization of the NVP particles can better preserve their electrical integration within the electrode, the loss of which is conjectured to be the major cause of capacity loss. The hypothesis was validated by performance comparison with a conventional carbon-coated NVP/C composite with the same initial discharge capacity but which showed a more severe capacity fading in extended cycling. This study presents not only a new strategy to extend the cycle life of cathode materials but also a cost-effective way to produce them

N–Co–O Triply Doped Highly Crystalline Porous Carbon: An Acid-Proof Nonprecious Metal Oxygen Evolution Catalyst

In comparison with nonaqueous Li–air batteries, aqueous Li–air batteries are kinetically more facile and there is more variety of non-noble metal catalysts available for oxygen electrocatalysis, especially in alkaline solution. The alkaline battery environment is however vulnerable to electrolyte carbonation by atmospheric CO₂ resulting in capacity loss over time. The acid aqueous solution is immune to carbonation but is limited by the lack of effective non-noble metal catalysts for the oxygen evolution reaction (OER). This is contrary to the oxygen reduction reaction (ORR) in acid solution where a few good candidates exist. We report here the development of a N–Co–O triply doped carbon catalyst with substantial OER activity in acid solution by the thermal codecomposition of polyaniline, cobalt salt and cyanamide in nitrogen. Cyanamide and the type of cobalt precursor salt were found to determine the structure, crystallinity, surface area, extent of Co doping and consequently the OER activity of the final carbon catalyst in acid solution. We have also put forward some hypotheses about the active sites that may be useful for guiding further work

Improving the Performance of High Capacity Li-Ion Anode Materials by Lithium Titanate Surface Coating

Current methods for improving the electrochemical performance of lithium-ion battery electrode materials mostly depend on materials design and synthesis. We propose that the unique electrochemical properties of spinel lithium titanate (Li₄Ti₅O₁₂, LTO) make it suitable as a protective coating to improve the performance of high capacity anode materials. In this study, tin oxide was coated with LTO to reduce the initial irreversible capacity loss because of solid electrolyte interface (SEI) formation and to improve the reversibility (capacity and rate performance) of tin oxide for Li⁺ storage. The LTO coating was applied to porous hollow tin oxide particles by a two-step process. Experimental measurements showed that the LTO coating shielded most of the direct contact between tin oxide and the electrolyte and hence the ICL due to SEI formation was reduced to mostly that of LTO, which is much lower than tin oxide. In addition the coated tin oxide also showed notable improvements in material cyclability and rate performance

Carbon-Encapsulated F‑Doped Li₄Ti₅O₁₂ as a High Rate Anode Material for Li⁺ Batteries

TiO₂ nanoparticles aggregated into a regular ball-in-ball morphology were synthesized by hydrothermal processing and converted to carbon-encapsulated F-doped Li₄Ti₅O₁₂ (LTO) composites (C-FLTO) by solid state lithiation at high temperatures. Through the careful control of the amount of carbon precursor (d(+)-glucose monohydrate) used in the process, LTO encapsulated with a continuous layer of nanoscale carbon was formed. The carbon encapsulation served a dual function: preserving the ball-in-ball morphology during the transformation from TiO₂ to LTO and decreasing the external electron transport resistance. The fluoride doping of LTO not only increased the electron conductivity of LTO through trivalent titanium (Ti³⁺) generation, but also increased the robustness of the structure to repeated lithiation and delithiation. The best-performing composite, C-FLTO-2, therefore delivered a very satisfying performance for a LTO anode: a high charge capacity of ∼158 mA h g^–1 at the 1 C rate with negligible capacity fading for 200 cycles and an extremely high rate performance up to 140 C

Fluoride-Assisted Synthesis of Plasmonic Colloidal Ta-Doped TiO₂ Nanocrystals for Near-Infrared and Visible-Light Selective Electrochromic Modulation

Dual-band electrochromic materials are integral to the development of smart windows where visible and near-infrared (NIR) light transmittance may be individually controlled. We present here colloidal Ta-doped TiO₂ anatase nanocrystals (NCs) as a promising candidate and their preparation by a fluoride-assisted synthesis method. The dual-band electrochromic performance of these NCs may be credited to their strong localized surface plasmon resonance (LSPR) absoption in the NIR region. The Ta doping of the TiO₂ NC host, which has not been attempted before, is made easy in the presence of the fluoride anions. The synthesis produces Ta-doped TiO₂ NCs as a highly uniform colloidal solution. Spectroscopic measurements indicate the generation of free carriers in the TiO₂ conduction band by the Ta⁵⁺ substitution of Ti⁴⁺ cations as the origin of the LSPR. Good dual-band electrochromic performance in terms of a high dynamic range for visible and near-infrared light modulation (86.3% at 550 nm and 81.4% at 1600 nm) and good electrochemical stability (the optical modulation at 550 and 1600 nm decreased by 1.3% and 6.7%, respectively, after 2000 cycles) were demonstrated in three-electrode cells to suggest Ta-doped TiO₂ NCs as a promising new electrode material for the smart windows

Observation of Cluster Size Growth in CO-Directed Synthesis of Au₂₅(SR)₁₈ Nanoclusters

The design of an efficient synthesis for large-scale production of atomically precise nanoclusters (NCs) is pivotal in realizing the size-dependent properties of the NCs. A simple and versatile method for producing atomically precise thiolated gold NCs (Au₂₅(SR)₁₈ NCs) in large quantities (∼200 mg) is demonstrated in this study. It uses a gaseous reducing agent, carbon monoxide (CO), to support a slow and size-controlled growth of Au₂₅(SR)₁₈ NCs. Absorption measurements of the reaction solution, which underwent distinct color changes (colorless → yellow → orange → brown → red-brown), allowed the formation of thiolated Au₂₅ NCs to be reconstructed from several key intermediates. The unique reaction environment provided by gaseous CO presents a new synthetic route to fabricate atomically precise metal NCs in quantities large enough for application explorations