Controllable Synthesis And Photocatalytic Performances Of Nanocrystallites SiC And TiO₂ Prepared By Arc Discharge Plasma

Ceramic/semiconductor materials became the research focus and hotspot,due to their great potentials in solving environmental pollution and energy crisis.Nanostructured semiconductor inherited the stability,oxidation resistance,high temperature resistance,chemical corrosion resistance and other excellent properties of the master bulk material,meanwhile the decrease of crystal size,increases of specific surface area and surface energy,etc.endow them novel properties such as photo/electrical catalytic properties,photoelectric conversions properties and electronic characteristics.Suitable band gap structure,excellent thermal stability,chemical stability,biocompatibility etc.of the wide band semiconductors of SiC and TiO₂ make them widely used in energy,environment and electronics and other fields.In general,the performances of nanomaterials greatly depend on their phase,morphology,size,surface state,crystal structure etc.,thus the study on controllable fabrication and related mechanism become significant in further researches.In this paper,the nanostructured precursors of SiC and TiO₂ with diverse morphologies and crystal structures were prepared by a simple route of DC arc plasma with merits of high efficiency,fast and low cost,thorugh controlling the electrode materials and the preparation atmosphere.Additionally,the chemical and physical modifications were adopted to further optimize the structure and properties of final products.The performances of energy conversion,i.e.optical responce,photocatalytic reaction and electrochemical reaction for the as-prepared precursors and modified products of SiC and TiO₂ nanocrystals,were also investigated.Using an on-line optical emission spectroscopy diagnosis,the formation mechanism in arc-discharge plasma for SiC nanocrystals was analysized.The experimental results favor to establish the corresponding relationships between reaction condition and resultant crystal structure/morphology as well the performance,and provide a theoretical basis for the design and control of nanostructured semiconductors.The main contents and conclusions of this dissertation are as follows:（1）Study on the controllable synthesis and formation mechanism of SiC nanocrystals.Experimental results show that the phases,structures and morphologies of SiC nanostructures can be controlled by the preparation atmospheres of DC arc-discharge plasma.The powders of Si/SiC,pure SiC,SiC/C nanocomposite were prepared by controlling CH4 concentration of atmosphere.The as-prepared SiC nanocrustals have hexagonal platelet-like morphology,with{111} crystal plane as the basal plane and {110} crystal plane as the side plane.SiC nanocrystals are encapsulated in carbon layers to form the hexagonal SiC@C nanocapsules（NCs）.The formation of SiC@C NCs contains the nucleation of truncated octahedral SiC cluster seed and the subsequent anisotropic growth of seed into a hexagonal nanoplatelet in the carbon-rich atmosphere.It is worth mentioning that H ions tend to suppress thermal graphitization,thus a thin and disordered carbon shell can be maintained on the surface of SiC@C NC.High-energy Ar plasma influences the energic status of reactants and can induce the formation of SiC polytye.Experimental results show that β-SiC（3C）is easily formed in the atmosphere with absence of argon or less argon gas,while α-SiC（6H）existes in a high concentration of Ar atmosphere.Additionally,the decrease of SiC crystal’s size and disapprence of its well-difined shape were found by rasing Ar content in the atmosphere.Boltzmann’s plot was used to estimate the plasma’s temperatures,which is 10582 K（in 2×104 Pa of Ar partial pressure）or 14523 K（in 4×104 Pa of Ar partial pressure）.It was also found that higher energy state of plasma favors the ionization of carbon atoms and promotes the formation of α-SiC polytype.（2）Structural evolution and photo/catalytic performance of SiC nanocrystals.By removing carbon shell from the surface of SiC@C NCs,it is demonstrated by UV-vis light absorbance that the direct bandgap of SiC nanocrystals is 5.72 eV and the indirect bandgaps of β-SiC（3C）and α-SiC（6H）are 3.13 eV and 3.32 eV,respectively.Degradation of MB pullutant under visible light was carried out and indicated that the photocatalytic kinetics are similar for the SiC nanopowders prepraed under increased CH4 concentrations,however their removal rates are different owing to the same pristine SiC cores in these powders.It indicates consequent that the SiC nanopowders prepared in different Ar contents will have the altered SiC cores and consequently show the different adsorption and photocatalytic performances.SiC@SiO₂ NCs were obtained by an interface evolution through a heat treatment at 650℃in air,during which the shape and inherent characteristic of pristine SiC core were inherited.Oxidation of surface silicon atoms on crystal SiC core can be restrained by existence of the carbon shell,and consequently resulted in a thin SiO₂ layer and a clear-cut hexagonal shape.The interface evolution from carbon to SiO₂ shell would endow SiC@SiO₂ NCs an enhanced photocatalytic activity,due to emerged hydrophilicity and transparentness of the SiO₂ shell.Additionally,the pure SiC nanoplates（SiC NPs）without any shells were prepared by stripping off SiO₂ shells from the SiC@SiO₂ NCs,using the etching effect of heated NaOH solution.（3）Synthesis of TiO₂ nanocrystals with TiN nanocrystals as the precursor.Firstly,TiN nanocrystals had been prepared by DC arc-diacharge plasma method,using metal titanium as titanium source and N2 as nitrogen source.The as-prepared TiN nanocrystals was hydrothermally converted to N-,S-and F-doped TiO₂,by emploing NaF and thioacetamide as F and S sources.The pure anatase-type TiO₂ powder was obtained by control molar ratio of titanium nitride,sodium fluoride and thioacetamide as 1:2:3（TO123）,setting the temperature as 200 ℃ and reaction for 36 h.XPS analysis showed that N,S and F elements are doped into the matrix of TiO₂ crystal.Additionally,some of anatase lattice can be converted to rutile-type phase with the proportion of sodium fluoride increased.The bandgap of samples TO100,TO120,TO103,TO123 and TO183 were 2.46 eV,2.56 eV,1.59 eV,3.02 eV,2.67 eV,3.24 eV,respectively,those are less than that of commercial TiO₂ powder product（P25）.Calculated positions through electronegativity for the conduction band and valence band of TO123 are-0.20 eV and +2.82 eV,respectively.Compared with the pure anatase the bottom of conduction band of TO123 shifts down,while the top of valence band shifts up.The photocatalytic activities of different samples were tested at optical wavelengthes greater than 420 nm.Samples TO123 and TO120 showed high photocatalytic activity and the removal rates for MB were 75%and 70%in 180 min.,respectively.（4）Preparation of pure TiO₂ nanopowder and TiO₂/WO3 nanocomposite powder by DC arc-discharge plasma method,their photocatalysis and electrochemical performances.Pure TiO₂ nanocrystals（T-TiO₂）and WO3/TiO₂ nanocomposites（W-TiO₂）were prepared by DC arc plasma method using Ti or W rod as the cathode,the environmental air as oxygen source and raw Ti bulk as the anode.The as-prepared nanopowders were spherical in particles’ morphology with the size of 20-40 nm.Some of WO3 exist as individual phase,but some of them matches to {002} crystal plane of TiO₂ with their {002} crystal plane.Both optical absorbances of two samples in visible range are higher than that of P25,due to the reason their bandgaps are 2.73 eV and 2.93 eV,respectively.The photocatalytic activities of different samples were investigated by employing MB as target pollutant under visible light wavelength larger than 420 nm.Both W-TiO₂ and Ti-TiO₂ samples showed higher photocatalytic activities,and the removal rates in 120 min.were 92%and 80%,respectively.Additionally,using Ti-TiO₂ and W-TiO₂ powders as the Li ions-active materials,the electrochemical performances of lithium-ion batteries’ anodes were studied.It was indicated that the anodes exhibit excellent cycle stability and high rate performance.A small amount of WO3 can enhance the electrochemical capacity,however the stability was slightly reduced.After 100 cycles,the discharge specific capacities of Ti-TiO₂ and W-TiO₂ electrodes become 130.8 mAh g’1 and 198.6 mAh g"1,with the coulombic efficiencies retained at 98%and 95%,respectively.