Research On The Microstructure And Mechanical Properties Of BN Nanotubes And Its Surface Activation For Long-range Motion And Subsequent Growth Of Au Nanocrystals

In situ transmission electron microscopy(TEM)technique provides the opportunity to observe at the atomic level materials responses to an external stimulus,discover transient states during chemical or structural transformations,and correlate materials structure to their functionalities.It has been widely used in material synthesis,energy materials,catalysis and life sciences and thus identified as one of the major future directions of electron microscopy.Boron nitride nanotubes(BNNTs)could be imagined as a carbon nanotubes(CNTs)in which C atoms were entirely substituted by alternating B and N atoms.BNNTs has been an important one-dimensional nanomaterials widely concerned because it possess high thermal conductivity,good temperature stability,insulation and mechanical properties.Research on the structure and properties of a single BNNTs by experiment has important basic research value.In this paper,we studied the structure of multi-walled BNNTs prepared by chemical vapor deposition.An then the structure evolution behavior of BNNTs with axial compression was studied by in situ TEM holder equipped with a force transducer(MEMS)holder.Then,Au nanoparticles were coated on the surface of BNNTs by physical sputterin.The behavior of Au nanoparticles on the surface of BNNTs under electron beam irradiation was studied.The underlying physical mechanism was discussed after systematic evaluating the fators which dominant the behavior of Au nanoparticles.The main contents of this paper include the following aspects:1.The micro-mechanical study on single multi-wall BN nanotube.The results shows that the multi-wall BN nanotube exhibit a single V-shaped kink under axial pressure.When the bending angle is less than 24.6 degrees,BN nanotubes exhibit excellent elasticity and flexibility.The high resolution image shows that the crystal structure at the fracture position is amorphous.The critical buckling force of BN nanotube is measured to 2124 n N.Young's modulus 1.29 TPa and elasticity coefficient 255.9 N/m of BN nanotube were calculated by Euler formula and Hooker's law,respectively.The reversible V-shape buckles indicated that these thermodynamically unstable B–B or N–N bonds(SW transformation)could switch back to stable B–N bonds after the compression force unloaded,due to the more energetically favorite BN nanotube structures.2.The physical sputtering method was employed to deposit Au nanoparticles on the BNNTs surface.The results show that Au nanoparticles are uniformly distributed on the surface of the BNNTs substrate and exhibit very narrow particle size distributions of dominated around 2-4 nm.The XPS results suggest that the interaction between Au nanoparticles and BNNTs is not chemical bonding but physical absorption via the Van Edward force.3.For the first time,BNNTs were used as the substrate of Au nanoparticles to study the behavior of Au nanoparticles under electron beam irradiation.Compared with the carbon or graphene films used as Au nanoparticle substrate in previous studies,the main advantage of BNNTs is its chemical inert,that is,no chemical bonding between BN and Au.It is the first time to observe the Au nanoparticles(2–4 nm)that experienced anomalous diffusive motion and subsequent crystal growth with surprisingly high rates on the BNNTs surface under beam irradiation.The high density static charging induced surface activation e.g.,atomic diffuse disorder and plasma,which were caused by the charger transfer between insulator BNNTs and Au nanocrystals under electron beam,is responsible for these behaviors and phenomena.4.A new physical mechanism was proposed to interpret the process,i.e.,the charge transfer,resulting in Au nanoparticles being gradually subject to be heavily electrified thus occurring the occasional rotation.The growth process firstly undergoes the lattice diffusion and subsequently the surface-dominated diffusion mechanism.These abnormal phenomena that metals carrying like charges enabled inter-attraction and motion suggest structural changes and evolution should be concerned for the real metal systems that are under electron charging.This work highlights the importance and proposes new insights in understanding the dynamics behaviors for real metal nanocrystal systems that are helpful in the exploration design,synthesis,etc.