Suman Neupane

In Situ Transmission Electron Microscopy Investigation of the Electrochemical Lithiation-Delithiation of Individual Co9S8/Co-Filled Carbon Nanotubes

Carbon nanotube (CNT)-encapsulated metal sulfides/oxides are promising candidates for application as anode materials in lithium ion battery (LIB), while their electrochemical behavior and mechanism still remain unclear. A comprehensive understanding of the lithiation mechanism at nanoscale of this type of composites will benefit the design and development of high-performance LIB materials. Here, we use Co9S8/Co nanowire-filled CNTs as a model material to investigate the lithium storage mechanism by in situ transmission electron microscopy. For a Co9S8/Co nanowire-filled closed CNT, the reaction front propagates progressively during lithiation, causing an axial elongation of 4.5% and a radial expansion of 32.4%, while the lithiated nanowire core is still confined inside the CNT. Contrastingly, for an open CNT, the lithiated Co9S8 nanowire shows an axial elongation of 94.2% and is extruded out from the open CNT. In particular, a thin graphite shell is drawn out from the CNT wall by the extruded lithiated Co9S8. The thin graphite shell confines the extruded filler and protects the filler from pulverization in the following lithiation-delithiation cycles. During multiple cycles, the Co segment remains intact while the Co9S8 exhibits a reversible transformation between Co9S8 and Co nanograins. Our observations provide direct electrochemical behavior and mechanism that govern the CNT-based anode performance in LIBs.

In Situ Transmission Electron Microscopy Observation of Electrochemical Sodiation of Individual Co9S8-Filled Carbon Nanotubes

The comprehension of fundamental electrochemical behavior and sodiation mechanism is critical for the design of high-performance electrode materials for sodium-ion (Na-ion) batteries. In this paper, the electrochemical sodiation process and microstructure evolution of individual Co9S8-filled carbon nanotubes (CNTs) have been directly visualized and studied using in situ transmission electron microscopy. Upon the first sodiation, a reaction front propagates progressively along the filling nanowire, causing the filled CNT to inflate. The filled CNTs behave differently depending on their structures and the magnitude of the sodiation voltage. For a Co9S8-filled CNT with an open end, the sodiated Co9S8 filler shows a substantial axial elongation of 120.8% and a small radial swelling due to the extrusion of CNT walls. In contrast, the closed CNT shows a major radial expansion of 40.6% and a small axial elongation because of the mechanical confinement of the carbon shells. After sodiation, the spacing between the carbon shells increases from 3.4 to 3.8 Å due to the Na(+)-ion insertion and the single-crystalline Co9S8 filler converts to numerous Co nanograins dispersed in a Na2S matrix. Compared with the gentle microstructure evolution of the CNT under small charging voltage, a strong electrochemical reaction accompanying drastic swelling and fracturing of CNT shells is observed for the CNT electrode under large charging voltage. Our observations provide direct evidence and important insights for the electrochemical process of CNT-based composite materials in Na-ion batteries.

Multiple Step Growth of Single Crystalline Rutile Nanorods with the Assistance of Self-Assembled Monolayer for Dye Sensitized Solar Cells

A novel multiple step growth (MSG) process has been developed to synthesize rutile nanorods (NRs) on fluorine-doped tin oxide (FTO) glass with the assistance of a self-assembled monolayer (SAM) aiming to increase the internal surface area of the 1D materials for dye sensitized solar cell (DSSC) applications. The experimental result reveals that the SAM layer can be selectively decomposed at the tip of the nanorod, namely the rutile (001) surface, due to the anisotropic photocatalytic property of the rutile. The remaining SAM layer on the side-wall of the NRs remains intact and serves as water repellent which prevents the radial growth of the NRs during the next step hydrothermal synthesis; therefore, the spacing between the NRs and the porosity of the NR array can be retained after additional growth cycles. On the other hand, introduction of a middle layer formed via TiCl4 solution treatment before the next growth cycle is found to be an effective way to control the diameters of the newly grown NRs. The performance of DSSC made from the rutile NRs grown using the MSG technique has been examined, and it is significantly affected by the internal surfaces of the NRs. Furthermore, the MSG combined with NR etching treatment by acid at low temperature (150 °C) leads to a significant enhancement in the solar cell performance. The gigantic wettability difference of the NRs before and after the SAM treatment as well as the MSG method could be adapted to prepare superhydrophobic and superhydrophilic nanostructured patterns for other applications.

Multilayered graphene acquires ferromagnetism in proximity with magnetite particles

Anisotropic diamagnetism of pristine graphite and graphene is well known. Here, evidence of significant induced ferromagnetism in multilayer graphene (MLG) decorated with ferrimagnetic Fe3O4 particles is reported. This MLG-Fe3O4 nano-composite was prepared by a one-step ultrasonic treatment at 75 °C in the surfactant sodium dodecyl-benzene-sulfonate. To verify the phase structure and morphology of the composite, X-ray diffraction, scanning and transmission electron microscopy, scanning tunneling electron microscopy, and Raman spectroscopy were employed. Room temperature data of magnetization versus magnetic field showed that the saturation magnetization MS = 58.6 emu/gm for pristine Fe3O4 increased to MS = 158.4 emu/gm for a 1:1 composite of Fe3O4 to MLG. These results lead to induced MS = 253 emu/gm in MLG resulting from its proximity to Fe3O4. Similar experiments on Fe3O4 to single walled carbon nanotubes (SWNT) composite did not show any induced magnetism in SWNT.

Nanowires of Fe/multi-walled carbon nanotubes and nanometric thin films of Fe/MgO

We observed that nanowires of Fe grown in the lumens of multi-walled carbon nanotubes required four times higher magnetic field strength to reach saturation compared to planar nanometric thin films of Fe on MgO(100). Nanowires of Fe and nanometric thin films of Fe both exhibited two fold magnetic symmetries. Structural and magnetic properties of 1-dimensional nanowires and 2-dimensional nanometric films were studied by several magnetometery techniques. The θ-2θ x-ray diffraction measurements showed that a (200) peak of Fe appeared on thin film samples deposited at higher substrate temperatures. In these samples prepared at higher temperatures, lower coercive field and highly pronounced two-fold magnetic symmetry were observed. Our results show that maximum magnetocrystalline anisotropy occurred for sample deposited at 100 °C and it decreased at higher deposition temperatures.

Synthesis and electron field emission of vertically aligned carbon nanotubes grown on stainless steel substrate

Vertically aligned carbon nanotube (CNT) array was synthesized directly on stainless steel (SS) substrates with or without chemical etching or catalyst deposition via plasma enhanced chemical vapor deposition method. The effect of chemical etching on the SS substrates resulted in CNTs with uniform diameter and length. Electron field emission properties of CNT arrays synthesized on pristine SS and Ni/Cr coated SS substrates were measured. The results indicated that CNTs synthesized on pristine SS substrate exhibited lower turn-on electric field, lower threshold electric field, and a higher field enhancement factor as compared to the CNTs synthesized on Ni/Cr coated SS. The robust contact between the CNTs and the SS substrate resulted in the improved stability during the field emission.