Jiqiu Wen

Controlling the quantity of α-Fe inside multiwall carbon nanotubes filled with Fe-based crystals: The key role of vapor flow-rate

The growth control of α-Fe inside multiwall carbon nanotubes has challenged researchers for more than a decade owing to the coexistence of this phase with Fe3C and γ-Fe. Previously, long heating treatments of 20 h have been used to decompose the encapsulated Fe-phases in C and Fe; however, these methods were limited by an unusual oxidation process leading to nanotube decomposition. In this letter, we report an alternative chemical vapour deposition approach that through an accurate control of the ferrocene-vapour flow-rate allows to achieve the direct encapsulation of 95% of α-Fe without additional heating treatments.

Cm-size free-standing self-organized buckypaper of bucky-onions filled with ferromagnetic Fe3C

Novel cm-size free-standing buckypapers of bucky-onions filled with a single-phase of ferromagnetic Fe3C single crystals were serendipitously discovered. These buckypapers are obtained directly in situ as the dominant product of the pyrolysis of ferrocene. Vibrating sample magnetometry also revealed an extremely large coercivity of 0.120 tesla and a saturation magnetization of 41 emu g−1.

Observation of large coercivities in radial carbon nanotube structures filled with Fe3C and FeCo single-crystals by viscous boundary layer pyrolysis of ferrocene and cobaltocene

Viscous boundary layer chemical vapor synthesis is a novel technique that uses the viscous boundary layer between a laminar pyrolysed metallocene/Ar vapor flow and a rough surface to induce the nucleation and growth of radial carbon nanotube (CNT) structures highly filled with ferromagnetic materials. Here we report the synthesis and characterization of radial structures comprising multiwall CNTs filled with large quantities of Fe3C and FeCo alloys and low quantities of γ-Fe in the forms of small single crystals. Surprisingly high saturation magnetizations up to 80 emu g−1 and a very high coercivity of 1400 Oe at room temperature are found. Such values of magnetization suggest that no room-temperature magnetic interaction is present between γ-Fe and the ferromagnetic crystals in our samples. The presence of such large coercivity values may be associated with the small size of the encapsulated particles which is strongly dependent on the high evaporation temperature of the precursors for fixed pyrolysis temperatures and vapour flow rate. The addition of Cl-species is also considered in the attempt to slow down the growth-rate of the radial CNT-structure and further investigate their growth mechanism.

Micrometre-length continuous single-crystalline nm-thin Fe3C-nanowires with unusual 010 preferred orientation inside radial few-wall carbon nanotube structures: the key role of sulfur in viscous boundary layer CVS of ferrocene

A key challenge in the fabrication of carbon nanotubes filled with ferromagnetic nanowires is the control of the number of nanotube-walls together with the nanowire continuity, composition and crystallinity. We report the serendipitous observation of novel radial carbon nanotube structures with few walls (2–5 walls) filled with nm-thin and many-micrometres long continuous single-crystalline Fe3C nanowires. These are the dominant reaction products in chemical vapour synthesis experiments involving the pyrolysis of ferrocene/sulfur mixtures in the viscous boundary layer between a rough surface and a laminar Ar flow. These nanowires are found with an unusual preferred 010 orientation along the nanotube capillary. The properties of these structures are investigated through the use of multiple techniques: SEM, TEM, HRTEM, EDX, STEM, XRD, Raman spectroscopy, FT-IR spectroscopy and VSM.

New insights on the dynamics of the γ-Fe/α-Fe phase-transition inside iron-filled carbon nanotubes

One of the challenges in the field of carbon nanotubes (CNTs) is the encapsulation of a single crystalline phase of ferromagnetic α-Fe. The formation of additional γ-Fe and Fe3C phases during CNT-growth generally limits the direct encapsulation of these crystals in the form of a single phase. A solution, the use of post-synthesis annealing, has been considered; however oxidation of the encapsulated metal-phases is commonly found due to the elevated temperatures (T) necessary for the phase-conversion. Here we investigate the dynamics of γ-Fe to α-Fe transition by T-dependent X-ray diffraction in vacuum. We show that a direct γ-Fe to α-Fe transition is present already below 200 °C and becomes significantly fast in the T-range of 300–399 °C. In such a T-range no metal oxidation is found. Rietveld refinement analyses also show that a T-dependent increase in the unit-cell c-axis value of the graphitic CNT-walls is present.

Peeling off effects in vertically aligned Fe3C filled carbon nanotubes films grown by pyrolysis of ferrocene

We report the observation of an unusual self-peeling effect which allows the synthesis of free standing vertically aligned carbon nanotube films filled with large quantities of Fe3C and small quantities of γ-Fe crystals. We demonstrate that this effect depends on the interplay of three main factors: (1) the physical interactions between the chosen substrate surface and grown carbon nanotubes (CNTs), which is fixed by the composition of the used substrate (111 SiO2/Si or quartz), (2) the CNT-CNT Van der Waals interactions, and (3) the differential thermal contraction between the grown CNT film and the used substrate, which is fixed by the cooling rate differences between the grown film and the used quartz or Si/SiO2 substrates. The width and stability of these films are then further increased to cm-scale by addition of small quantities of toluene to the ferrocene precursor.

Room temperature synthesis of ReS2 through aqueous perrhenate sulfidation

In this study, a direct sulfidation reaction of ammonium perrhenate (NH4ReO4) leading to a synthesis of rhenium disulfide (ReS2) is demonstrated. These finding reveal the first example of a simplistic bottom-up approach to the chemical synthesis of crystalline ReS2. The reaction presented here takes place at room temperature, in an ambient and solvent-free environment and without the necessity of a catalyst. The atomic composition and structure of the as-synthesized product were characterized using several analysis techniques including energy dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, thermogravimetric analysis (TGA) and differential scannig calorimetry (DSC). The results indicated the formation of a lower symmetry (1Td) ReS2 with a low degree of layer stacking.

Enhanced magnetization in unusual carbon-nanotube/carbon-foam cm-scale hybrid-buckypaper films with high α-Fe filling-ratio

We report the synthesis of novel and unusual α-Fe-filled carbon nanotube (CNT)/carbon foam (CFM) hybrid-buckypaper films via pyrolysis of ferrocene/dichlorobenzene mixtures. The presence of a direct connection between the CFM and the CNT-buckypapers is found to significantly enhance the magnetization properties revealing room temperature saturation magnetizations as high as 73 emu g−1. The magnetic properties of these films are compared to those typical of cm-scale horizontally aligned and randomly oriented Fe3C-filled CNTs buckypapers and to those observed for Fe-filled and Ni-filled CFMs. This finding has a great significance in the field of ferromagnetically filled CNTs, since it implies that the properties of α-Fe-filled CFM can be combined with those of ferromagnetically filled CNTs in a single buckypaper-structure for possible future application as a microwave absorber. The properties of the obtained films are characterized in detail with scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and vibrating sample magnetometry.

Cl-Assisted Large Scale Synthesis of Cm-Scale Buckypapers of Fe3C-Filled Carbon Nanotubes with Pseudo-Capacitor Properties: The Key Role of SBA-16 Catalyst Support as Synthesis Promoter

We show a novel chemical vapour deposition (CVD) approach, in which the large-scale fabrication of ferromagnetically-filled cm-scale buckypapers is achieved through the deposition of a mesoporous supported catalyst (SBA-16) on a silicon substrate. We demonstrate that SBA-16 has the crucial role of promoting the growth of carbon nanotubes (CNTs) on a horizontal plane with random orientation rather than in a vertical direction, therefore allowing a facile fabrication of cm-scale CNTs buckypapers free from the onion-crust by-product observed on the buckypaper-surface in previous reports. The morphology and composition of the obtained CNTs-buckypapers are analyzed in detail by scanning electron microscopy (SEM), Energy Dispersive X-ray (EDX), transmission electron microscopy (TEM), high resolution TEM (HRTEM), and thermogravimetric analysis (TGA), while structural analysis is performed by Rietveld Refinement of XRD data. The room temperature magnetic properties of the produced buckypapers are also investigated and reveal the presence of a high coercivity of 650 Oe. Additionally, the electrochemical performances of these buckypapers are demonstrated and reveal a behavior that is compatible with that of a pseudo-capacitor (resistive-capacitor) with better performances than those presented in other previously studied layered-buckypapers of Fe-filled CNTs, obtained by pyrolysis of dichlorobenzene-ferrocene mixtures. These measurements indicate that these materials show promise for applications in energy storage systems as flexible electrodes.

UV absorption investigation of ferromagnetically filled ultra-thick carbon onions, carriers of the 217.5 nm Interstellar Absorption Feature

Carbon nano-onions (CNOs) are fullerene-like structures which consist of quasi-spherical closed carbon shells. These structures have become a subject of great interest thanks to their characteristic absorption feature of interstellar origin (at 217.5 nm, 4.6 μm−1). An additional extinction peak at 3.8 μm−1 has also been reported and attributed to absorption by graphitic residues between the as-grown CNOs. Here, we report the ultraviolet absorption properties of ultra-thick CNOs filled with FePt3 crystals, which also exhibit two main absorption peaks—features located at 4.58 μm−1 and 3.44 μm−1. The presence of this additional feature is surprising and is attributed to nonmagnetic graphite flakes produced as a by-product in the pyrolysis experiment (as confirmed by magnetic separation methods). Instead, the feature at 4.58 μm−1 is associated with the π-plasmonic resonance of the CNOs structures. The FePt3 filled CNOs were fabricated in situ by an advanced one-step fast process consisting in the direct subli...