Yun Zong

Multicolor Barcoding in a Single Upconversion Crystal

We report the synthesis of luminescent crystals based on hexagonal-phase NaYF4 upconversion microrods. The synthetic procedure involves an epitaxial end-on growth of upconversion nanocrystals comprising different lanthanide activators onto the NaYF4 microrods. This bottom-up method readily affords multicolor-banded crystals in gram quantity by varying the composition of the activators. Importantly, the end-on growth method using one-dimensional microrods as the template enables facile multicolor tuning in a single crystal, which is inaccessible in conventional upconversion nanoparticles. We demonstrate that these novel materials offer opportunities as optical barcodes for anticounterfeiting and multiplexed labeling applications.

Dual-Phase Spinel MnCo2O4 and Spinel MnCo2O4/Nanocarbon Hybrids for Electrocatalytic Oxygen Reduction and Evolution

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential reactions for energy-storage and -conversion devices relying on oxygen electrochemistry. High-performance, nonprecious metal-based hybrid catalysts are developed from postsynthesis integration of dual-phase spinel MnCo2O4 (dp-MnCo2O4) nanocrystals with nanocarbon materials, e.g., carbon nanotube (CNT) and nitrogen-doped reduced graphene oxide (N-rGO). The synergic covalent coupling between dp-MnCo2O4 and nanocarbons effectively enhances both the bifunctional ORR and OER activities of the spinel/nanocarbon hybrid catalysts. The dp-MnCo2O4/N-rGO hybrid catalysts exhibited comparable ORR activity and superior OER activity compared to commercial 30 wt % platinum supported on carbon black (Pt/C). An electrically rechargeable zinc-air battery using dp-MnCo2O4/CNT hybrid catalysts on the cathode was successfully operated for 64 discharge-charge cycles (or 768 h equivalent), significantly outperforming the Pt/C counterpart, which could only survive up to 108 h under similar conditions.

Co3O4 nanoparticle-modified MnO2 nanotube bifunctional oxygen cathode catalysts for rechargeable zinc–air batteries

We report the preparation of MnO2 nanotubes functionalized with Co3O4 nanoparticles and their use as bifunctional air cathode catalysts for oxygen reduction reaction and oxygen evolution reaction in rechargeable zinc-air batteries. These hybrid MnO2/Co3O4 nanomaterials exhibit enhanced catalytic reactivity toward oxygen evolution reaction under alkaline conditions compared with that in the presence of MnO2 nanotubes or Co3O4 nanoparticles alone.

One‐Pot Synthesis of Tunable Crystalline Ni3S4@Amorphous MoS2 Core/Shell Nanospheres for High‐Performance Supercapacitors

Transition metal sulfides gain much attention as electrode materials for supercapacitors due to their rich redox chemistry and high electrical conductivity. Designing hierarchical nanostructures is an efficient approach to fully utilize merits of each component. In this work, amorphous MoS(2) is firstly demonstrated to show specific capacitance 1.6 times as that of the crystalline counterpart. Then, crystalline core@amorphous shell (Ni(3)S(4)@MoS(2)) is prepared by a facile one-pot process. The diameter of the core and the thickness of the shell can be independently tuned. Taking advantages of flexible protection of amorphous shell and high capacitance of the conductive core, Ni(3)S(4) @amorphous MoS(2) nanospheres are tested as supercapacitor electrodes, which exhibit high specific capacitance of 1440.9 F g(-1) at 2 A g(-1) and a good capacitance retention of 90.7% after 3000 cycles at 10 A g(-1). This design of crystalline core@amorphous shell architecture may open up new strategies for synthesizing promising electrode materials for supercapacitors.

Two-Dimensional Tin Disulfide Nanosheets for Enhanced Sodium Storage.

Sodium-ion batteries (SIBs) are considered as complementary alternatives to lithium-ion batteries for grid energy storage due to the abundance of sodium. However, low capacity, poor rate capability, and cycling stability of existing anodes significantly hinder the practical applications of SIBs. Herein, ultrathin two-dimensional SnS2 nanosheets (3-4 nm in thickness) are synthesized via a facile refluxing process toward enhanced sodium storage. The SnS2 nanosheets exhibit a high apparent diffusion coefficient of Na(+) and fast sodiation/desodiation reaction kinetics. In half-cells, the nanosheets deliver a high reversible capacity of 733 mAh g(-1) at 0.1 A g(-1), which still remains up to 435 mAh g(-1) at 2 A g(-1). The cell has a high capacity retention of 647 mA h g(-1) during the 50th cycle at 0.1 A g(-1), which is by far the best for SnS2, suggesting that nanosheet morphology is beneficial to improve cycling stability in addition to rate capability. The SnS2 nanosheets also show encouraging performance in a full cell with a Na3V2(PO4)3 cathode. In addition, the sodium storage mechanism is investigated by ex situ XRD coupled with high-resolution TEM. The high specific capacity, good rate capability, and cycling durability suggest that SnS2 nanosheets have great potential working as anodes for high-performance SIBs.

Enhanced Surface Plasmon Resonance on a Smooth Silver Film with a Seed Growth Layer

This paper reports an effective method to enhance the surface plasmon resonance (SPR) on Ag films by using a thin Ni seed layer assisted deposition. Ag films with a thickness of about 50 nm were deposited by electron beam evaporation above an ultrathin Ni seed layer of approximately 2 nm on both silicon and quartz substrates. The root-mean-square (rms) surface roughness and the correlation length have been reduced from >4 nm and 28 nm for a pure Ag film to approximately 1.3 and 19 nm for Ag/Ni films, respectively. Both experimental and simulation results show that the Ag/Ni films exhibit an enhanced SPR over the pure Ag film with a narrower full width at half-maximum. Ag films with a Ge seed layer have also been prepared under the same conditions. The surface roughness can be reduced to less than 0.7 nm, but narrowing of the SPR curve is not observed due to increased absorptive damping in the Ge seed layer. Our results show that Ni acts as a roughness-diminishing growth layer for the Ag film while at the same time maintaining and enhancing the plasmonic properties of the combined structures. This points toward its use for low-loss plasmonic devices and optical metamaterials applications.

Influence of carbon pore size on the discharge capacity of Li–O2 batteries

Porous carbon materials play key roles in rechargeable Li–O2 batteries as oxygen diffusion media and sites for reversible electrode reactions. Despite tremendous efforts in the synthesis of various porous carbon materials, the influence of carbon materials on cell capacity remains unclear. Based on our study of eight different carbon electrode materials with various pore sizes and pore volumes in Li–O2 batteries, we found that the initial discharge capacity was hardly affected by the surface area or pore volume. Instead, it was directly correlated with the pore sizes. To further verify this finding, meso- and macro-porous carbon materials with pore sizes in the range of 20 to 100 nm were prepared using spherical silica as a template. The results clearly showed that the cell capacity increases with the increase of pore size and eventually reached its maximum at 7169 mA h g−1 at a pore size of 80 nm. A physical model proposed to illustrate the influence of carbon pore size on cell capacity is the formation of a monolayer of Li2O2 with a thickness of 7.8 nm inside the carbon pores during the discharge process which limits the diffusion of incoming oxygen at smaller pore size (<80 nm).

Potential of metal-free "graphene alloy" as electrocatalysts for oxygen reduction reaction

Extensive research and development on theoretical calculation and synthetic methods over the past few years have made doped graphene one of the most promising candidates for metal-free oxygen reduction reaction (ORR) catalysts. However, from the performance point of view, there is still a long way to go for these doped graphene-based catalysts to meet the requirements needed for commercial applications. What is the key to further improve the catalytic activity of doped graphene toward ORR to make them commercially viable? In this review, we will try to answer this question by fundamentally giving a detailed analysis based on the theoretical calculations to reveal the origin of ORR activity of doped graphene and the structure–performance relationship of such materials. Thereafter, we will provide an overview on the recent advances in the catalytic activity improvement of doped graphene, including major works using approaches of increasing the number of active sites, controlling the doping types (particularly for nitrogen doped graphene), developing co-doped graphene, and extending the surface area of doped graphene. Finally, in this perspective, we discuss some development opportunities and pathways that can lead to more efficient doped-graphene based ORR electrocatalysts approaching the practical use for fuel cells and metal–air batteries.

One-Pot Synthesis of Highly Anisotropic Five-Fold-Twinned PtCu Nanoframes Used as a Bifunctional Electrocatalyst for Oxygen Reduction and Methanol Oxidation.

Five-fold-twinned PtCu nanoframes (NFs) with nanothorns protruding from their edges are synthesized by a facile one-pot method. Compared to commercial Pt/C catalyst, the obtained highly anisotropic five-fold-twinned PtCu NFs show enhanced electrocatalytic performance toward the oxygen reduction reaction and methanol oxidation reaction under alkaline conditions.

Sulfur–carbon yolk–shell particle based 3D interconnected nanostructures as cathodes for rechargeable lithium–sulfur batteries

We report for the first time a novel method to synthesize sulfur–carbon yolk–shell particles with sulfur fully confined inside the conductive carbon shells, and the filling content of sulfur can be well-controlled and fine-tuned. In the yolk–shell structure the sulfur spheres partially occupy the internal void space of highly conductive carbon, allowing for comfortable accommodation of the volume expansion of sulfur upon lithiation during the battery discharge process. 3D interconnected nanostructures based on such sulfur–carbon yolk–shell particles exhibit a high initial capacity of 560 mA h g−1 (per gram electrode) with good cycling performance, promising high potential in rechargeable lithium–sulfur batteries for a wide range of applications.