玻璃中CdSeS纳米晶体的室温光致发光谱

对掺有过饱和的镉、硒和少量硫的玻璃在500～800℃分别退火4h,生长了不同尺寸的CdSe_1-xS_x纳米晶体。测量了纳米晶体的室温吸收光谱和光致发光(PL)光谱,550℃生长的样品在300～800nm的范围没有观察到吸收和发光峰,表明温度低于550℃玻璃中不能形成纳米晶体。生长温度在600～650℃,纳米晶体的PL光谱主要为两个宽的发光带,即带边激子发光带和通过表面态复合的发光带。随着生长温度的升高,带边复合发光的蓝移减小,通过表面态的发光逐渐消失,并出现了叠加于宽发光带上的一系列明显的弱发射峰。不同温度生长的样品中,叠加峰的能量相同。同一样品中叠加峰的能量不随激发光波长的变化而变化。     

Colloidal Crystals of NaYF4 Upconversion Nanocrystals Studied by Small‐Angle X‐Ray Scattering (SAXS)

Spherical NaYF₄ upconversion nanocrystals with mean radii of about 5 and 11 nm are observed to form colloidal crystals, i.e., 3D assemblies of the particles with long‐range order. The colloidal crystals of the larger particles form directly in solution when dispersions of the particles in toluene are stored at room temperature for several weeks. Crystallization of the smaller particles takes place when their dispersions in hexane are slowly dried at elevated temperatures. The formation and the structure of the colloidal crystals are studied by small‐angle X‐ray scattering (SAXS). SAXS measurements show that the smaller as well as the larger particles assemble into a face‐centered cubic lattice with unit cell dimensions of a = 18.7 nm and a = 35.5 nm, respectively. The SAXS data also show that the particles in the colloidal crystals still bear a layer of oleic acid on their surfaces. The thickness of this layer is 1.5–1.8 nm, as determined by comparing the unit cell dimensions of the colloidal crystals with the mean particle sizes. The latter could be very precisely determined from the distinct oscillations observed in the SAXS data of dilute colloidal dispersions of the nanocrystals.     

Full Protection for Graphene‐Incorporated Micro‐/Nanocomposites Containing Ultra‐small Active Nanoparticles: the Best Li‐Storage Properties

In recent years, graphene‐incorporated micro‐/nanocomposites represent one of the hottest developing directions for the composite materials. However, a large number of active nanoparticles (NPs) are still in the unprotected state in most constructed graphene‐containing designs, which will seriously impair the effects of the graphene additives. Here, a fully protected Fe₃O₄‐based micro‐/nanocomposite (G/Fe₃O₄@C) is rationally developed by carbon‐boxing the common graphene/Fe₃O₄ microparticulates (G/Fe₃O₄). The processes and results of full protection are tracked in detail and characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy, and nitrogen absorption–desorption isotherms, as well as scanning and transition electron microscopy. When used as the anode for lithium‐ion batteries, the fully protected G/Fe₃O₄@C exhibits the best lithium‐storage properties in terms of the highest rate capabilities and the longest cycle life compared to the common G/Fe₃O₄ composites and commercial Fe₃O₄ products. These much improved properties are mainly attributed to its novel structural features including complete protection of active Fe₃O₄ nanoparticles by the surface carbon box, a robust conductive network composed of nitrogen‐doped graphene nanosheets, ultra‐small Fe₃O₄ NPs of 4–5 nm, abundant mesopores to accommodate the volume variation during cycling, and micrometer‐sized secondary particles.     

Towards Excellent Anodes for Li‐Ion Batteries with High Capacity and Super Long Lifespan: Confining Ultrasmall Sn Particles into N‐Rich Graphene‐Based Nanosheets

Sn is regarded as a promising anode material for Li‐ion batteries due to high capacity and cost effectiveness. Hitherto large‐scale fabrication of Sn‐based materials while achieving both high capacity and long cycle life remains challenging, but it is highly required for its realization in practical applications. Furthermore, low melting point always casts shadow over the morphology‐controllable preparation, and leads to multistep or high‐cost processes. Here, a facile and scalable method is devised for a 2D hybrid structure of Sn@graphene‐based nanosheets incorporating of optimized nitrogen species (≈13 wt%). Distinct from conventional Sn–C composites, the fairly N‐rich carbon nanosheets liberate limited potential of low N doping, induce massive extra Li‐storage sites, and encourage a high capacity significantly. In addition, these abundantly anchored heteroatoms also promote the homogeneous dispersion and robust confinement of ultrasmall Sn nanoparticles into the flexible graphene‐based nanosheets. This elastic encapsulation towards Sn nanoparticles admirably maintains structural integrity through effective remission of volume expansion, demonstrating a super long‐term cyclic stability for 1000 cycles. This structural and componential engineering offers a significant implication for rational design of materials in extended areas of energy conversion and storage.     

Metal Oxide Nanomaterials with Nitrogen‐Doped Graphene‐Silk Nanofiber Complexes as Templates

Fabricating electrode materials with superior electrochemical performance remains a challenge. Here, a simple but effective strategy for the formation of metal oxide nanomaterials with superior performance has been developed. Silk protein nanofibers adhered on reduced graphene oxide (rGO) sheets are used as templates to regulate the formation of nanostructured iron oxide composites, achieving porous nanorod structures that could not be attained in control experiments. These porous nanorods demonstrate superior electrochemical performance as electrodes with retention of a capacity of 1495 mAh g^?1 after 180 cycles at 0.2 C and a rate capability of 900 mAh g^?1 at 2 C discharge rate. These new rGO/silk composite templates provide a more controllable environment for Fe₂O₃ fabrication, resulting in improved nanostructures and superior electrical performance. The strategy developed here should also be more broadly applicable in the design of metal oxide nanomaterials with specialized structures and useful performance.     

Oleylamine‐Assisted Phase‐Selective Synthesis of Cu2−xS Nanocrystals and the Mechanism of Phase Control

Environmentally friendly Cu_2?x S compounds exist in many different mixed phases in nature, while their nanoscale counterparts can be pure phase with interesting localized surface plasmon resonance properties. Because of the complexity of composition and phase, controllable synthesis of Cu_2?x S nanocrystals becomes an important scientific issue in colloidal chemistry. In this work, a hot‐injection method is developed to synthesize Cu_2?x S nanocrystals by injecting a sulfur precursor into a copper precursor using oleylamine and octadecene as solvents. By varying the reaction parameters (temperature, volume ratio of oleylamine/octadecene, molar ratio of Cu/S in the precursors), hexagonal CuS, monoclinic Cu_1.75S, and rhombohedral Cu_1.8S, nanocrystals can be selectively synthesized, providing a platform to illustrate the mechanism of crystal phase control. The crystal phase control of Cu_2?x S nanocrystals is oleylamine‐determined by controlling the molar ratio of Cu/S in the reaction precursors as well as the ratio of Cu_2?x S clusters/Cu⁺ in the subsequent reaction. More importantly, temperature plays an important role in varying the molar ratio of Cu/S and Cu_2?x S clusters/Cu⁺ in the reaction system, which significantly influences the crystal phase of the resulting Cu_2?x S nanocrystals. The understanding into crystal control provides a guideline to realize reproducible phase‐selective synthesis and obtain well‐defined high‐quality materials with precise control.     

Formation of Sn@C Yolk–Shell Nanospheres and Core–Sheath Nanowires for Highly Reversible Lithium Storage

As one promising anode material with high theoretical capacity, metallic tin has attracted much research interest in the field of lithium‐ion batteries. Here, two types of tin/carbon (Sn@C) core–shell nanostructures with inner buffering voids are fabricated from SnO₂ hollow nanospheres via a facile chemical vapor deposition (CVD) method. The crystallinity and surface topography of SnO₂ hollow nanospheres are found to affect the morphology of resultant Sn@C materials. Sn@C yolk–shell nanospheres and core–sheath nanowires are obtained from the as‐prepared SnO₂ and high‐temperature annealed SnO₂ nanospheres, respectively. The unique Sn@C nanostructures can mitigate the agglomeration/pulverization of Sn nanoparticles and electrical disconnection from the current collector caused by the large volume change during the lithium alloying/dealloying process. Both Sn@C yolk–shell and core–sheath nanostructures show stable cycling performance up to 500 cycles with specific capacities of ca. 430 and 520 mA h g^?1, respectively.     

Electrospinning Synthesis of Porous NiCoO2 Nanofibers as High‐Performance Anode for Lithium‐Ion Batteries

Nanostructured ternary/mixed transition metal oxides have attracted considerable attentions because of their high‐capacity and high‐rate capability in the electrochemical energy storage applications, but facile large‐scale fabrication with desired nanostructures still remains a great challenge. To overcome this, a facile synthesis of porous NiCoO₂ nanofibers composed of interconnected nanoparticles via an electrospinning–annealing strategy is reported herein. When examined as anode materials for lithium‐ion batteries, the as‐prepared porous NiCoO₂ nanofibers demonstrate superior lithium storage properties, delivering a high discharge capacity of 945 mA h g^?1 after 140 cycles at 100 mA g^?1 and a high rate capacity of 523 mA h g^?1 at 2000 mA g^?1. This excellent electrochemical performance could be ascribed to the novel hierarchical nanoparticle‐nanofiber assembly structure, which can not only buffer the volumetric changes upon lithiation/delithiation processes but also provide enlarged surface sites for lithium storage and facilitate the charge/electrolyte diffusion. Notably, a facile synthetic strategy for fabrication of ternary/mixed metal oxides with 1D nanostructures, which is promising for energy‐related applications, is provided.     

Hollow Silica Spheres Embedded in a Porous Carbon Matrix and Its Superior Performance as the Anode for Lithium‐Ion Batteries

Silica (SiO₂) is regarded as one of the most promising anode materials for lithium‐ion batteries due to the high theoretical specific capacity and extremely low cost. However, the low intrinsic electrical conductivity and the big volume change during charge/discharge cycles result in a poor electrochemical performance. Here, hollow silica spheres embedded in porous carbon (HSS–C) composites are synthesized and investigated as an anode material for lithium‐ion batteries. The HSS–C composites demonstrate a high specific capacity of about 910 mA h g^?1 at a rate of 200 mA g^?1 after 150 cycles and exhibit good rate capability. The porous carbon with a large surface area and void space filled both inside and outside of the hollow silica spheres acts as an excellent conductive layer to enhance the overall conductivity of the electrode, shortens the diffusion path length for the transport of lithium ions, and also buffers the volume change accompanied with lithium‐ion insertion/extraction processes.     

Fabrication of Cu@MxOy (M = Cu,Mn, Co,Fe) Nanocable Arrays for Lithium‐Ion Batteries with Long Cycle Lives and High Rate Capabilities

A new strategy is reported to fabricate Cu@M_xO_y (M = Cu, Mn, Co, Fe) nanocable arrays using five‐fold twinned copper (Cu) nanowire (NW) arrays as starting materials, to promote both the cycling stability and high rate capability of M_xO_y as anodes for LIBs. Conductive Cu NW arrays were synthesized on Cu foil via chemical vapor deposition (CVD), followed by the oxidation of their surface so as to form Cu@Cu₂O nanocable arrays. The thickness of the active material (Cu₂O) on the Cu NW arrays can be tuned from 20 nm to 160 nm by simply controlling the oxidation time. Based on this accurate control, the optimal coating thickness of Cu₂O was determined to be around 35 nm. Additionally, the Cu₂O active material shell can be easily transformed to other metal oxides with even higher specific capacities via a “coordinating etching” strategy based on Pearson's principle, resulting in Cu@M_xO_y nanocable arrays (M = Mn, Co, Fe). When applied as electrodes for LIBs, these 3D electrodes show long cycle lives (over 300 cycles) and high rate capabilities.     

Synthesis of Porous NiO Nanorods as High‐Performance Anode Materials for Lithium‐Ion Batteries

1D nanostructured metal oxides with porous structure have drawn wide attention to being used as high‐performance anode materials for lithium‐ion batteries (LIBs). This study puts forward a simple and scalable strategy to synthesize porous NiO nanorods with the help of a thermal treatment of metal‐organic frameworks in air. The NiO nanorods with an average diameter of approximately 38 nm are composed of nanosized primary particles. When evaluated as anode materials for LIBs, an initial discharge capacity of 743 mA h g^?1 is obtained at a current density of 100 mA g^?1, and a high reversible capacity is still maintained as high as 700 mA h g^?1 even after 60 charge–discharge cycles. The excellent electrochemical performance is mainly ascribed to the 1D porous structure.     

Controllable Formation of Niobium Nitride/Nitrogen‐Doped Graphene Nanocomposites as Anode Materials for Lithium‐Ion Capacitors

Niobium nitride/nitrogen‐doped graphene nanosheet hybrid materials are prepared by a simple hydrothermal method combined with ammonia annealing and their electrochemical performance is reported. It is found by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) that the as‐obtained niobium nitride nanoparticles are about 10–15 nm in size and homogeneously anchored on graphene. A non‐aqueous lithium‐ion capacitor is fabricated with an optimized mass loading of activated carbon cathode and the niobium nitride/nitrogen‐doped graphene nanosheet anode, which delivers high energy densities of 122.7–98.4 W h kg^?1 at power densities of 100–2000 W kg^?1, respectively. The capacity retention is 81.7% after 1000 cycles at a current density of 500 mA g^?1. The high energy and power of this hybrid capacitor bridges the gap between conventional high specific energy lithium‐ion batteries and high specific power electrochemical capacitors, which holds great potential applications in energy storage for hybrid electric vehicles.     

Strategies to curb structural changes of lithium/transition metal oxide cathode materials & the changes' effects on thermal & cycling stability

Structural transformation behaviors of several typical oxide cathode materials during a heating process are reviewed in detail to provide in-depth understanding of the key factors governing the thermal stability of these materials. We also discuss applying the information about heat induced structural evolution in the study of electrochemically induced structural changes. All these discussions are expected to provide valuable insights for designing oxide cathode materials with significantly improved structural stability for safe, long-life lithium ion batteries, as the safety of lithium-ion batteries is a critical issue; it is widely accepted that the thermal instability of the cathodes is one of the most critical factors in thermal runaway and related safety problems.     

Facile Fabrication of Bicomponent CoO/CoFe2O4‐N‐Doped Graphene Hybrids with Ultrahigh Lithium Storage Capacity

A facile strategy is developed to fabricate bicomponent CoO/CoFe₂O₄‐N‐doped graphene hybrids (CoO/CoFe₂O₄‐NG). These hybrids are demonstrated to be potential high‐performance anodes for lithium‐ion batteries (LIBs). The CoO/CoFe₂O₄ nanoplatelets are finely dispersed on the surface of N‐doped graphene nanosheets (CoO/CoFe₂O₄‐NG). The CoO/CoFe₂O₄‐NG electrode exhibits ultrahigh specific capacity with 1172 mA h g^?1 at 500 mA g^?1 and 970 mA h g^?1 at 1000 mA g^?1 as well as excellent cycle stability due to the synergetic effects of N‐doped graphene and CoO/CoFe₂O₄ nanoplatelets. The well‐dispersed bicomponent CoO/CoFe₂O₄ is responsible for the high specific capacity. The N‐doped graphene with high specific surface area has dual roles: to provide active sites for dispersing the CoO/CoFe₂O₄ species and to function as an electrical conducting matrix for fast charge transfer. This method provides a simple and efficient way to configure the hybridized electrode materials with high lithium storage capacity.     

Eggshell‐Assisted Preparation of Mixed Nanoparticles Simultaneously Encapsulated in Carbon Microcubes for Reversible Lithium Storage

Nanoparticle‐based electrodes often suffer from poor electrical properties due to high interparticle resistance, as well as low Coulombic efficiency attributed to large surface area induced parasitic reactions. In order to address this issue, a strategy of encapsulating two kinds of nanoparticles of both metal oxide and metallic nanoparticles is attempted, simultaneously, in microscale carbon cubic shells for highly reversible lithium storage. The unique structure is synthesized by simultaneous reactions of (1) decomposition of crystalline Co₂(OH)₃Cl microparticle precursor, synthesized in unique eggshell reactor systems, into nanoparticles, (2) partial reduction of CoO into metallic Co by eggshell membrane, (3) carbon coating by chemical vapor deposition facilitated by presence of catalytic Co with carbon released from the eggshell membrane, and (4) microscale carbon shell formed using the Co₂(OH)₃Cl particles as microtemplates. The carbon shells can prevent the encapsulated mixed nanoparticles from direct contact with electrolyte and reduce undesirable parasitic reactions, and accommodate volumetric variation during cycling. The introduction of metallic Co nanoparticles can reduce interparticle resistance. When evaluated for lithium storage, the unique structures of CoO–Co@C demonstrate superior electrochemical performances in terms of electrode stability and rate performance, as compared to that of pure CoO.     

High Rate and Long Cycle Life of a CNT/rGO/Si Nanoparticle Composite Anode for Lithium‐Ion Batteries

Si nanoparticle (Si‐NP) composite anode with high rate and long cycle life is an attractive anode material for lithium‐ion battery (LIB) in hybrid electric vehicle (HEV)/pure electric vehicle (PEV). In this work, a carbon nanotube (CNT)/reduced graphene oxide (rGO)/Si nanoparticle composite with alternated structure as Li‐ion battery anode is prepared. In this structure, rGO completely wraps the entire Si/CNT networks by different layers and CNT networks provide fast electron transport pathways with reduced solid‐state diffusion, so that the stable solid‐electrolyte interphase layer can form on the whole surface of the matrix instead of on single Si nanoparticle, which ensure the high cycle stability to achieve the excellent cycle performance. As a result, the CNT/rGO/Si‐NP anode exhibits high performances with long cycle life (≈455 mAh g^?1 at 15 A g^?1 after 2000 cycles), high specific charge capacity (≈2250 mAh g^?1 at 0.2 A g^?1, ≈650 mAh g^?1 at 15 A g^?1), and fast charge/discharge rates (up to 16 A g^?1). This nanostructure anode with facile and low‐cost synthesis method, as well as excellent electrochemical performances, makes it attractive for the long life cycles at high rate of the next generation LIB applications in HEV/PEV.