Fast Ionic Conductor Catalyzed Growth,Mechanism And Properties Investigations Of Metastable MnE (E=S,Se) Low-Dimensional Nanostructures

The catalytic growth and catalytic kinetics of low-dimensional nanostructures are always the active topic in the areas associated with crystal growth,condensed-matter physics,semiconductor physics and device,solid-state chemistry and materials science.Metal(base)-catalyzed vapor-liquid-solid(VLS)growth mechanism/model and its derivatives including solution-liquid-solid(SLS)and vapor-solid-solid(VSS)growth method were successful for fabrication of one-dimensional nanostructures.Recently,solution-solid-solid(SSS)mechanism based on the fast ionic conductor catalysis extends the growth route and research scope of low-dimensional nanostructures.Although metastable low-dimensional nano materials have aroused wide attention because of their special structures and unique properties,how to synthesize the metastable low-dimensional nanomaterials via the SSS mechanism has not been reported up to now.The present thesis will focus on developing the SSS growth method based on the fast ionic conductor catalysis and extending its application in controllable growth of metastable(non-thermodynamic stable equilibrium state)semiconducting low-dimensional nanostructures,heteronanostructures and solid-state solution.In addition,we investigated and verified the structure,microstructure,growth mechanism and kinetic stabilizing mechanism of the obtained low-dimensional nanomaterials.The main research results and innovations of this thesis are summarized as follows:1.A novel catalytic growth strategy via the solution-solid-solid(SSS)mechanism to grow metastable zincblende MnSe nanowires was established and developed.The controllable preparation of metastable zincblende MnSe nanowires was achieved.The first growth of four-coordinated metastable zincblende MnSe ultralong nanowires catalyzed by fast ionic conductor Ag2Se via SSS growth mechanism under mild solution phase reaction condition at 120-200 ?.In the present reaction system,body-centered cubic(bcc)phase Ag2Se with fast ionic conductor structure performed high catalytic ability during the catalytic growth of ultralong MnSe nanowires even in a very low concentration.The MnSe nanowires were grown along the<110>zone axis rather than the commonly observed<111>direction for a typical four-coordinated Grimm-Sommerfeld bonding solid.The investigation suggested that the formation of catalyst solid-state solution composition,the existence and transfer of solid-solid interface between catalyst and nanowire stem exhibited the direct evidence for the solid state catalyst catalyzed the growth of the nanowires.For realizing and understanding of the catalytic growth of the MnSe nanowires over the bcc-Ag2Se catalyst,we performed TEM investigations using longtime election beam illumination on the end part of nanowire with catalyst tip to reappear the catalytic growth process/kinetics and reveal the mechanism of the nanowire growth directly.Investigations showed that incorporated source material of MnSe within the catalyst tip(a form of solid-state solution between MnSe stem and Ag2Se catalyst)sustained the growth of the nanowire continuously in the catalyst-nanowire solid-solid interface in that the nanowire grown forward with the catalyst-nanowire interface moving ahead through source material transport due to the exceptional catalytic ability associated to the superionic conducting structure of the bcc-Ag2Se catalyst with high-density vacancies and fast mobility of silver(I)cations.The present solid-solid interface catalysis via the SSS mechanism through taking advantage of solid-state fast ion diffusion/transport would enhance the anisotropically epitaxial growth of the MnSe nanowires in metastable structure,kinetically,as compared to liquid-solid interface catalysis.Meanwhile,the performed doping strategy associated with solid-state solution formation in addition to the use of mild synthetic conditions also favored the growth and stabilization of the metastable nanowires in kinetic aspects,generally.SSS growth model can also be used for growth of ultrathin MnSe nanowires,Ag2Se-MnSe nanorods/matchstick-like heteronanostructures and even untralong MnS nanowires.Although the dynamics in the actual growth process is complicated,this research provides some useful reference and new revelation to realize the direct catalytic growth and kinetic stabilization of one-dimensional nanostructures under mild reaction conditions.2.The novel method of catalytic growth of metastable zincblende MnS nanowires was developed.First growth of the zincblende phase MnS nanowires and Ag2S-MnS heteronanowires catalyzed by fast ionic conductor via solution-solid-solid(SSS)mechanism under mild colloidal solution phase reaction.MnS is stable thermodynamically as six-coordinate cubic rock-salt form under ambient conditions,and four-coordinated metastable are more likely to form as wurtzite nanostructure because of its higher ionic bonding,namely it would be more difficult to obtain MnS in zincblende phase compared with MnSe.The current study realized the controllable growth of metastable zincblende MnS nanowires due to the phase transformation and catalysis of Ag2S from low-temperature monoclinic phase to body-centered cubic fast ionic conducting phase.Intensive investigations demonstrated that the metastable zincblende MnS nanowires were grown by the real catalysis of bcc-Ag2S no matter what kind of Ag sourced was introduced into the current reaction system.However,Mn sources had not any obvious influence on the morphology and phase of the grown zincblende MnS nanowires.The obtained MnS nanowires had interesting structure/microstructure.Most of them were grown along the<112>zone axis rather than the commonly observed<111>direction for a typical four-coordinated Grimm-Sommerfeld bonding solid(It was also different from the<110>direction of MnSe nanowires.),there were still some ones in small proportion grown along different directions,which along the zincblende<111>direction/wurtzite<0001>direction were bent/twist with short segments presenting as zincblende/defect-section or wurtzite/defect-section superlattices connected with the MnS stem along<112>direction.Meanwhile,the investigation showed that the structure and stability of the nanowires sensitively affected by reaction temperature,reaction time even reaction pressure and so on:elevating reaction temperature and prolonging reaction time promoted the transition of the metastable MnS nanowires to the corresponding thermodynamic rock-salt form accompanied with the change of the shape.The increase of the reaction pressure was favorable for metastable stabilization of wurtzite MnS nanowires.Such as low-pressure variation did not have an obvious effect on a condensed matter in common,while even in small-scale increase(1-2 atm)considerably influenced the growth of the MnS nano wires from zincblende to wurtzite form in the present catalytic system.The paper studied and proved the metastable catalytic growth mechanism and the structure transformation mechanism of the four-coordinated MnS nanowires.The investigation enriched the realization of metastable manganese chalcogenides and provided the valuable reference for exploring the catalytic growth of other one-dimensional metastable nanostructures.3.A controllable synthetic route for metastable orthorhombic phase AgGaSe2 nanocrystals and its solid-state solution nanostructures was expanded.We successfully prepared AgGaSe2 and AgIn1-xGaxSe2(0?x?1)nanocrystals.Metastable orthorhombic AgGaSe2 nanocrystals were obtained from a mild solution strategy.Conditional experiments displayed that we can obtain this metastable structure at 200 ? to 270 ? for 1 to 120 min.200 ? is the minimum temperature and 1 min is the shortest reaction time for synthesizing orthorhombic AgGaSe2 nanocrystals.What's more,the morphology of orthorhombic AgGaSe2 nanocrystals can be tuned from spherical nanoparticles to water-drop-like nanoparticles via regulating reaction temperature.In addition,we ascertained the growth mechanism of the metastable orthorhombic AgGaSe2 nanocrystals through conditional experiments.Moreover,we prepared In substituted orthorhombic AgGaSe2 solid-state solution quaternary AgIn1-xGaxSe2(0?x?1)nanocrystals with tunable composition and optical band gap.The optical band gap can be tuned from 1.24 eV(x=0)to 1.79/1.81 eV(x=1.0),and linear fitting showed the nearly linear relationship between the composition x and the optical band gap energies of the AgIn1-xGaxSe2 nanocrystals.It provided a simple and useful route to prepare other new metastable orthorhombic phase ?-?-?2 nanostructures of Ag-based ternary and quaternary chalcogenides,such as AgGaS2,AgGa(S1-xSex)2,AgIn1-xGaxS2,AgInTe2,AgGaTe2,AgIn1-xGaxTe2 nanocrystals under mild condition.