Wan-Jing Yu

Improved electrochemical performance of Fe2O3 nanoparticles confined in carbon nanotubes

A hybrid material of carbon nanotube (CNT)-encapsulated Fe2O3 nanoparticles was prepared by immersing CNTs with two open ends in a Fe(NO3)3 solution followed by thermal decomposition. It was found that the hollow core of the CNTs was filled with a homogeneous array of Fe2O3 nanoparticles with each nanoparticle being a single crystal. As an anode material of lithium-ion batteries, the Fe2O3-filled CNTs exhibited an improved electrochemical performance in terms of high reversible capacity, excellent cycling stability (811.4 mA h g−1 after 100 cycles), and high rate capability, compared to that of pure Fe2O3. We attribute this superior electrochemical performance of the Fe2O3-filled CNTs to the small size of the Fe2O3 nanoparticles, the confinement effect of CNTs, sound electrical contact between these two components, as well as the good electrical conductivity and unique porous structure of CNTs that improve the electron and lithium ion transport ability of the anode.

Importance of oxygen in the metal-free catalytic growth of single-walled carbon nanotubes from SiOx by a vapor-solid-solid mechanism

To understand in-depth the nature of the catalyst and the growth mechanism of single-walled carbon nanotubes (SWCNTs) on a newly developed silica catalyst, we performed this combined experimental and theoretical study. In situ transmission electron microscopy (TEM) observations revealed that the active catalyst for the SWCNT growth is solid and amorphous SiO(x) nanoparticles (NPs), suggesting a vapor-solid-solid growth mechanism. From in situ TEM and chemical vapor deposition growth experiments, we found that oxygen plays a crucial role in SWCNT growth in addition to the well-known catalyst size effect. Density functional theory calculations showed that oxygen atoms can enhance the capture of -CH(x) and consequently facilitate the growth of SWCNTs on oxygen-containing SiO(x) NPs.

Lithiation of Silicon Nanoparticles Confined in Carbon Nanotubes

Silicon has the highest theoretical lithium storage capacity of all materials at 4200 mAh/g; therefore, it is considered to be a promising candidate as the anode of high-energy-density lithium-ion batteries (LIBs). However, serious volume changes caused by lithium insertion/deinsertion lead to a rapid decay of the performance of the Si anode. Here, a Si nanoparticle (NP)-filled carbon nanotube (CNT) material was prepared by chemical vapor deposition, and a nanobattery was constructed inside a transmission electron microscope (TEM) using the Si NP-filled CNT as working electrode to directly investigate the structural change of the Si NPs and the confinement effect of the CNT during the lithiation and delithiation processes. It is found that the volume expansion (∼180%) of the lithiated Si NPs is restricted by the wall of the CNTs and that the CNT can accommodate this volume expansion without breaking its tubular structure. The Si NP-filled CNTs showed a high reversible lithium storage capacity and desirable high rate capability, because the pulverization and exfoliation of the Si NPs confined in CNTs were efficiently prevented. Our results demonstrate that filling CNTs with high-capacity active materials is a feasible way to make high-performance LIB electrode materials, taking advantage of the unique confinement effect and good electrical conductivity of the CNTs.

Carbon nanotube-clamped metal atomic chain

Metal atomic chain (MAC) is an ultimate one-dimensional structure with unique physical properties, such as quantized conductance, colossal magnetic anisotropy, and quantized magnetoresistance. Therefore, MACs show great potential as possible components of nanoscale electronic and spintronic devices. However, MACs are usually suspended between two macroscale metallic electrodes; hence obvious technical barriers exist in the interconnection and integration of MACs. Here we report a carbon nanotube (CNT)-clamped MAC, where CNTs play the roles of both nanoconnector and electrodes. This nanostructure is prepared by in situ machining a metal-filled CNT, including peeling off carbon shells by spatially and elementally selective electron beam irradiation and further elongating the exposed metal nanorod. The microstructure and formation process of this CNT-clamped MAC are explored by both transmission electron microscopy observations and theoretical simulations. First-principles calculations indicate that strong covalent bonds are formed between the CNT and MAC. The electrical transport property of the CNT-clamped MAC was experimentally measured, and quantized conductance was observed.

Visualizing the roles of graphene for excellent lithium storage

Graphene has been extensively used in hybrid electrodes for its notable improvement of lithium storage properties. However, direct visualization of the roles of graphene and the origin for the enhancement at the nanoscale are highly inadequate, which are difficult to be obtained by ex situ methods. Here, we use in situ transmission electron microscopy to visualize the roles of graphene during lithiation using a NiO/graphene hybrid as a model material. We witness that graphene has three roles in a strong-coupled NiO/graphene hybrid: (1) it increases the Li+ diffusion rate by two orders of magnitude; (2) it strongly improves Li+ reaction kinetics with NiO at high current densities and facilitates the homogeneous lithiation of NiO; (3) it severely restricts the expansion of NiO near the interface, ensuring stable electrical contact between graphene and NiO during extended cycling. Combined with the electrochemical measurements and first-principles calculations, this study further verifies the interface-induced graphene enhancement and distinctly provides valuable insights for excellent lithium storage by constructing interfacial binding between graphene and active materials to make full use of graphene.

Template synthesis of ultra-thin and short carbon nanotubes with two open ends

Anodic aluminium oxide (AAO) films containing small diameter channels with both ends open were prepared for use as a growth template for carbon nanotubes (CNTs). The smallest channel diameter was 7 nm and the smallest film thickness was 2 mu m. The CNTs produced from the AAO template by chemical vapor deposition of acetylene had controllable outer and inner diameters and length, and open ends. The pore texture, structure and crystallinity of the CNTs were characterized by electron microscopy, nitrogen gas adsorption-desorption, and thermogravimetric analysis. The inner diameters of the CNTs were adjustable by selecting AAO templates with specific channel diameters and/or tuning the duration of chemical vapor deposition. CNTs with an outer diameter, inner diameter and length of 7 nm, 4.5 nm and 2 mu m, respectively, were obtained, the narrowest and shortest ever reported for CNTs with two open ends. These thin, short, and open CNTs would render easy access, filling and transport of foreign matter into their one-dimensional cavity with prominent confinement effects.

Synthesis and Electrochemical Lithium Storage Behavior of Carbon Nanotubes Filled with Iron Sulfide Nanoparticles.

Carbon nanotubes (CNTs) filled with iron sulfide nanoparticles (NPs) are prepared by inserting sulfur and ferrocene into the hollow core of CNTs followed by heat treatment. It is found that pyrrhotite‐11T iron sulfide (Fe‐S) NPs with an average size of ≈15 nm are encapsulated in the tubular cavity of the CNTs (Fe‐S@CNTs), and each particle is a single crystal. When used as the anode material of lithium‐ion batteries, the Fe‐S@CNT material exhibits excellent electrochemical lithium storage performance in terms of high reversible capacity, good cyclic stability, and desirable rate capability. In situ transmission electron microscopy studies show that the CNTs not only play an essential role in accommodating the volume expansion of the Fe‐S NPs but also provide a fast transport path for Li ions. The results demonstrate that CNTs act as a unique nanocontainer and reactor that permit the loading and formation of electrochemically active materials with desirable electrochemical lithium storage performance. CNTs with their superior structural stability and Li‐ion transfer kinetics are responsible for the improved rate capability and cycling performance of Fe‐S NPs in CNTs.

Amorphization and Directional Crystallization of Metals Confined in Carbon Nanotubes Investigated by in Situ Transmission Electron Microscopy.

The hollow core of a carbon nanotube (CNT) provides a unique opportunity to explore the physics, chemistry, biology, and metallurgy of different materials confined in such nanospace. Here, we investigate the nonequilibrium metallurgical processes taking place inside CNTs by in situ transmission electron microscopy using CNTs as nanoscale resistively heated crucibles having encapsulated metal nanowires/crystals in their channels. Because of nanometer size of the system and intimate contact between the CNTs and confined metals, an efficient heat transfer and high cooling rates (∼10(13) K/s) were achieved as a result of a flash bias pulse followed by system natural quenching, leading to the formation of disordered amorphous-like structures in iron, cobalt, and gold. An intermediate state between crystalline and amorphous phases was discovered, revealing a memory effect of local short-to-medium range order during these phase transitions. Furthermore, subsequent directional crystallization of an amorphous iron nanowire formed by this method was realized under controlled Joule heating. High-density crystalline defects were generated during crystallization due to a confinement effect from the CNT and severe plastic deformation involved.