In Situ TEM Study Of Catalyst-free Growth Of One Dimensional Tungsten Oxide Nanomaterials

As the basis of one dimensional(1D)nanomarterials researches,the study of the growth mechanisms of 1D nanomartierals including nanowire and nanotube are the research hotspots.Gas phase method with the merit of fabricating almost all inorganic 1D nanomaterials has attracted numerious researchers.Recent years,by the use of in situ transmission electron microscopy(TEM),siganificant progresses concerning the growth mechanisms of catalyst-aided nanowires and nanotubes have been achieved,and the growth thermodynamics and kinetics of catalyst-aided nanowire and nanotube are becoming well understood.By contrast,the growth mechanism of cayalyst-free 1D nanomaterials remains elusive because of the lack of systematic in situ TEM investigation.In consideration of the above background,we chose W18O49,which is one of the most studied tungsten oxide,as our study object.We successfully grew noncatalytic W18O49 nanowire and nanotube in an environmental TEM through thermal oxidation method.We systematically studied the growth mechanism of catalyst-free nanowire and nanotube by the combination of in situ TEM,crystal thermodynamic equilibrium theory of crystal and First-principles calculation.The inovative results are listed as follows:(1)We successfully grew noncatalytic W18O49 nanowire in TEM by thermal oxidation of tungsten filament,and directly demonstrated the vapor-solid(VS)mechanism of W18O49 nanowire by a novel experiment of thermal oxidizing molybdenum affixed with tungsten lamella.In situ high resolution TEM observation indicated that the W18O49 nanowire grew by following the classical 2D nucleation and growth way.In particular,nanowire grew monomolecular by monomolecular via step nucleation and flow on the tip terrace of nanowire,accompanied by oscillatory mass transport from tip rim to tip terrace.Morphology prediction based on BFDH(Bravais-Friedel-Donnay-Harker)theory showed that the length of W18O49 crystal in[010]direction was much larger than that in other directions,in accordance with the experiment results.First-principles calculation of the energy of different W18O49 facets revealed that the(010)facet under proper O2 atmosphere had the highest energy,indicating that 2D nucleation on this facet with the smallest energy barrier was most favorable,thus nanowire would like to grew along[010]direction.(2)We explored the relationship between nanowire length and growth time by in situ TEM and found that the nanowire experienced two different growth periods,i.e.initially quick nonlinear growth and late slow linear growth.In addition,the real-time effect of the variation of factors such as O2 pressure,temperature,sidewall step and electron beam density on the nanowire growth kinetics were studied by in situ TEM.We also introduced autocorrelation analysis to investigate the correlation between adjacent nucleation events on nanowire tip terrace,and the results indicated that adjacent nucleation events in the nanowire growth were independent with each other,which was quite different with the case in vapor-liquid-solid(VLS).The possible reason was thought to be that 2D nucleation in VS was dominated directly by the external source pressure,while in VLS 2D nucleation was dominated by the chemical potential of the droplet.(3)We found the noncatalytic growth of W18O49 nanotubes in the region of the tungsten filament under higher oxygen pressure in TEM.In situ TEM investigation was conducted to investigate the growth danamic of W18O49 nanotube,and the real time observation indicated that the W18O49 nanotube was formed via the epitaxial growth of steps on the sidewall of leader W18O49 nanowire,which was different from the nanowires coalescence mechanism proposed previously.Based on the in situ TEM observations,we proposed the sidewall epitaxy growth mechanism for the W18O49 nanotube,i.e.under high saturability growth conduction,nanowire sidewall step nucleation and flow were vigorous,and the whole crystal might adjust its shape to tube-like structure in the requirement of heat dissipation via surface diffusion.