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1) Novel synthesis of BN nanotube by a substitution reaction method
A method involving carbon nanotubes substituted reaction was developed for the synthesis of mass quantities of boron nitride nanotubes. Boron oxide vapor was reacted with nitrogen gas in the presence of carbon nanotubes to form boron nitride nanotubes, whose diameters and lengths are similar to those of the starting carbon nanotubes. It is proposed that carbon atoms of carbon nanotubes can be fully substituted by boron and nitrogen atoms through a general chemical reaction. The successful preparation of BN-NTs with the average diameters and lengths similar to the starting CNTs confirms the applicability of this method—CNTS substituted reaction. Although the final BN-NTs nanotubes do not contain carbon element, CNTs may play an important role as precursors and in particular supply the skeletons for the growth of BN-NTs during the substitution reaction. After fully substituting the carbon atoms of CNTs by B and N atoms, the B and N atoms of BN-NTs may be involved in a rearrangement process for more suitable and ordered positions at high temperature. Although a deeper understanding of the BN-NTs growth mechanism is clearly needed, we note that this simple and cheap CNTs substituted reaction technology could in principle produce BN-NTs of any quantity. Only sufficient amount of CNT is needed, which have been synthesized in a large quantity and are commercially available. The CNTs substituted reaction developed here is easily accessible to others with an interest in BN-NTs and may also be extended to form nanotubes of other novel materials (for instance, the BxNyCz or C–BN–C sandwich structure nanotubes). The ready availability of large amounts of these nanotubes should offer more opportunities for both new fundamental science and useful nanotechnological applications.
2) Novel synthesis of BN nanosheet by a chemical blowing method
We have developed a new route of chemical blowing for the production of atomically thin (one-to-few atomic layers) free-standing BN and Cx–BN nanosheets. This technology has two main characteristic features: high volume yield and laterally large areas. These not only provide large enough flakes that are readily available for electrical and mechanical performance explorations, but also give enough nanosheet mass for fabricating ultimately strong polymeric or other composites. Significant mechanical reinforcement in PC/BN composites and delicate tuning of nanosheet conductivity through modifying C contents in them are demonstrated in this work. Additionally, this catalyst- and substrate-free method is simpler than a normal CVD method and its products generally possess larger lateral dimensions than those obtained during solution-exfoliation methods. The developed technique opens up a wide horizon for the analogous growth of other 2D nanosheets and full realization of their potentials in nanotechnology.
3) In-situ mechanical/electrical property measurements of BN nanomaterials under a 300kVTEM-STM/AFM
3.1 Deformation-driven carrier transport BN nanotube
In contrast to standard metallic or semiconducting graphitic carbon nanotubes, for years their structural analogs, boron nitride nanotubes, in which alternating boron and nitrogen atoms substitute for carbon atoms in a graphitic network, have been considered to be truly electrically insulating due to a wide band gap of layered BN. Alternatively, here, we show that under in situ elastic bending deformation at room temperature inside a 300 kV high-resolution transmission electron microscope, a normally electrically insulating multiwalled BN nanotube may surprisingly transform to a semiconductor. The semiconducting parameters of bent multiwalled BN nanotubes squeezed between two approaching gold contacts inside the pole piece of the microscope have been retrieved based on the experimentally recorded I-V curves. In addition, the first experimental signs suggestive of piezoelectric behavior in deformed BN nanotubes have been observed. Remarkably, the nanotube electrical transport properties were found to be smoothly tuned from insulating to semiconducting through a bending deformation. Resultantly, it was confirmed that the unmatched transition in the BNNT electrical performance had been governed by deformation. I-V hysteresis existed in bent BNNTs under the change in a bias voltage sweeping polarity, which could be a sign of BNNT piezoelectricity. Keeping in mind that BNNTs possess excellent mechanical properties and are ultralightweight materials, the discovered deformation-driven tuning of BNNT electrical appearance and piezoelectricity may have many interesting prospective applications in the nanoscale sensors, actuators, and advanced NEMS devices with integrated electronic/optical functions.
3.2 Direct force measurement at bending of BN nanotube
Individual multiwalled boron nitride nanotubes of different diameters (40−100 nm) were bent inside a 300 kV high-resolution transmission electron microscope (TEM) using a new fully integrated TEM−atomic force microscope (AFM) piezodriven holder under continuous recording of force−piezodisplacement curves. The tubes were gently compressed in situ (i.e., inside the electron microscope) between a piezomovable aluminum wire and a silicon cantilever. Typically, bending stress values ranging from ∼100 to ∼260 MPa, and corresponding to elastic moduli of 0.5−0.6 TPa, were estimated. Tube gross failures were absent up to very large bending angles (in excess of 115°). Extending the bending angles beyond 30−40° resulted in the elastic deformation of BN nanotubes, which proceeded through the propagation of consecutive momentary kinks. These had the effect of accumulating a bending curvature rather than uniformly curl the tube under the compression load. These kinks were found to be entirely reversible on reloading with no (or marginal) traces of residual plastic deformation.
3.3 Tensile tests on individual BN nanotube
Tensile loading and pullouts of individual multi‐walled BNNTs were studied for the first time by in‐situ tests in the integrated HRTEM‐AFM setup. Most important mechanical parameters, such as the ultimate tensile strength and strain, maximum sustainable load and the Young’s modulus of tubes, were measured. Under parallel HRTEM observations the measured mechanical parameters were correlated to experimental conditions and tube structures. Tensile loading and pullouts of individual multiwalled BNNTs were studied for the fi rst time by in-situ tests in the integrated HRTEM-AFM setup, and the measured mechanical parameters were correlated to experimental conditions and tube structures. All the reported mechanical properties, i.e., continuous shell breakage during pullout, lowering measured breaking strength, unique multishell breakage, are suggested to reflect the partially ionic character of the B-N bonds.
4) Mesoporous metallic nanoarchitectures with concave features
Combining the intrinsic catalytic properties of metallic materials, especially transition metals, with mesoporous structures would expect the improved catalytic activity as well as their stability towards many kinds of applications, including electrochemical energy conversion applications. We employed the soft-templating methods to synthesis various mesoporous metals, i.e., Au, Pt, and Pd, in particles and films structure. The soft templates were formed by micellization of surfactant or block copolymer in an aqueous solution. In the preparation of mesoporous nanoparticles, the metal precursors were reduced into a metallic state through the chemical reduction process. While in the preparation of mesoporous films, the metal ions were reduced along with the formation of a metallic framework due to the electrochemical deposition process on the conductive substrate (Au-coated Si wafer). The obtained mesoporous particles and films show a concave feature that contains a high abundance of step and kink sites. Those step and kink sites reflected the high-index facets which are preferable for various electrocatalytic reactions. For example, our calculation study shows that the oxidative species, such as -OH, have higher binding energy towards the high-index facet than that of the low-index facet. The number of bonded -OH on the catalyst surface is one of the rate-determining steps in the ethanol oxidation reaction. Our micelle-assembly approach can be extended to alloys systems and pave the way towards other functional promising applications, including direct alcohol fuel cells and non-enzymatic electrochemical glucose sensors.
Professor Yoshio Bando has been working on the synthesis of 1D/2D inorganic nanomaterials and detailed analysis of their atomic structures and properties using methods of advanced electron microscopy. He discovered many new nanotubes and nanowires, such as those made of BN, MgO, AlN and others. One particular highlight was the discovery of a carbon nanotube-based "nanothermometer", which is listed in the Guinness book of world records as the world's smallest thermometer. Professor Bando’s research group is a world pioneer in the large-scale BN nanotube fabrication and clarified many unique properties of BN nanotubes for the first time. In particular, his group has succeeded in producing polymer composite thin films containing BN nanotubes that show excellent mechanical and thermal properties. Recently, he and his colleagues have pursued nano-property measurements inside a Transmission Electron Microscope (TEM) and performed the first ever direct bending and tensile tests on BN nanotubes. All these delicate experiments allowed them to get a clear structure-property relationship down to the nanoscale of the materials dimensions, which is the Holy Grail of the whole Materials Science.