| Nanostructured semiconductors are new artifacts with unique physical and chemical properties,e.g.quantum size effect,interference effect,tunneling effect,coulomb blocking,nonlinear optical effect,photoelectric catalysis,photoelectric conversions,etc.They exhibit potentials in single electron storages,energy storage and conversions,quantum lasers and catalysis,etc.,and would fetch new openings for the advance of science and technology.In this work,the Si nanocrystals with sphere,sheet and ribbon morphologies,the TiC/C nanocrystals with cubic,truncated-cubic and truncated-octahedral rmorphologies,have been synthesized by direct current(DC)arc-discharge plasma method,in diverse atmospheres based on H2 and inert gases.In this synthesis route,high-energetic inert gas atoms could induce the nucleation of crystal seeds and further anisotropic growth,while hydrogen atoms can homogenize the surface energy of crystal seeds benefiting an isotropic growth.These peculiar effects in arc discharge make it different from the traditional preparation methods.Formation rules and mechanisms for the nanostructured semiconductors have been studied concerning the relationships among reaction atmospheres,compositions,crystal structures and morphologies.The experimental results and theoretical analysis favor the design and preparation of nanostructured semiconductors,as well to control their structures and physical/chemical properties.The photo/electro chemical characteristics,i.e.photoelectric response,electrochemical reactions and catalysis,of Si,SiC and TiC/C nanocrystals were investigated.The relationship between crystal habits and performances of semiconductor nanocrystals were also discussed.It is indicated that the morphologies of Si nanocrystals can be controlled by the atmospheres in DC arc plasma.Si nanocrystal is spherical particle if the atmosphere was of pure H2 gas,while two-dimensional Si nanosheet can be obtained in mixture atmosphere of H2 and Ar,whereas Si nanoribbon was fabricated in the atmosphere of H2 and He.In the arc-discharge process,the high-energetic H+ ions could adsorb on each facet of Si seed to identify their surface energies,which attributes equal growth rate of each facet and consequent isotropic growth of Si nanocrystals,i.e.Si nanoparticles were formed in this case.After adding inert gas(Ar or He)into H2 atmosphere,an anisotropic growth of Si crystal seed can be induced by the ionized inert atoms,accordingly the two-dimensional Si nanosheets were obtained in existence of Ar,and the one-dimensional Si nanoribbons were synthesized in He-containing atmosphere due to a second growth of fine ribbons can be promoted in this case.It is suggested that the fine Si pieces in two-dirmension can further grow into nanoribbons along<112>crystallographic direction of Si.Above three types of silicon nanomaterials(particle,sheet and ribbon)are the core-shell structure with very thin SiOx shell and Si core.By normalizing the SiOx content with the respective specific surface area of three kinds of Si nanocrystals,the densities of SiOx shells on Si nanosheets and Si nanoribbons are three times larger than that of Si nanoparticles.It also can be the evidence for higher surface activities of Si nanosheets and nanoribbons.The direct bandgaps of Si nanoparticles,nanosheets and nanoribbons were detected as 2.89 eV,2.92 eV and 3.02 eV,respectively,those are obviously larger than that of bulk silicon.As illumination by ultraviolet light,the photocurrents of Si nanoparticles,nanosheets and nanoribbons are 0.062,0.286 and 0.081 mA g-1,respectively.Si nanosheets have the strongest ultraviolet photoelectric response,indicating the small size and two-dimensional structure favor the photo/electric transition.On the basis of above pure Si nanocrystals,the nanostructures of binary Si-C system were synthesized and investigated.SiC composite nanopowders were prepared by arc-discharge in pure methane(0.005 and 0.01 MPa).It is found that the mixture of 3C-SiC,6H-SiC and Si was obtained under 0.005 MPa of methane(SiC0.005MPa),while 0.01 MPa of methane can promote carbonization(between Si and carbon)and less Si impurity was remained in SiC0.01MPa powders.Both SiC-based nanopowders were used as the testing catalysts to degrade a refractory 2,4-dichlorophenol through photoelectrocatalysis.The results show that the photoelectric removal efficiency reaches 92.5%for SiC0.01MPa catalyst,while no obvious efficiency was found for SiC0.005MPa catalyst.Obviously,the quality,e.g.components,crystal structures and morphologies of SiC-based nanopowders greatly affect the photoelectrocatalysis performances.Semiconductor carbide TiC was also studied in this work,on the basis of Si-C binary system.Core-shell type TiC/C nanocrystals with various morphologies have been successfully synthesized by arc-discharge plasma through regulating contents of H2 and Ar gases in the reaction atmospheres.TiC nanocrystals exhibit multiple morphologies as synthesis in pure methane,as a result of the free growth of TiC seeds.In the atmosphere case of Ar and methane,TiC/C nanocrystals are cubic with {100} facets as the exposure facets,while they are truncated-octahedral with {100} and {111} exposure facets by adding H2 gas.If adding both of H2 and Ar into methane atmosphere,the TiC/C nanocrystals product becomes truncated-cubic structures.Complicated phenomena in the morphologies of TiC/C nanocrystals have been well explained by the effect of high-energetic H+ ions on TiC seeds,i.e.to uniform surface energies of seed’s facets and induce an isotropic growth.On the contrary,Ar+ ions promote an anisotropic growth of TiC seeds to form the cubic structures.TiC nanocrystals are truncated-cubic under the synergistic effects of H+ and Ar+ ions.The carbon shell on surface of TiC/C nanocrystals is graphite-like layers with plentiiul lattice defects.TiC/C nanocrystal samples have been used as the cathode catalysts to degrade 2,4-dichlorophenol through electro-catalytic reduction.It is indicated that the removal efficiencies of samples A-D reach to 60%,66%,70%and 73%at bias potentials of-1.17,-1.17,-1.12,-0.8 V,respectively,in 180 minutes.The removal efficiency was improved by increasing the area of(111)facet in one TiC/C nanocrystal,ascribed to the benefits from morphology of nanocrystal catalysis.By four times repeating operations with sample D,the removal efficiency was still higher than 64%,implying the TiC-core is well protected by the carbon-shell of TiC/C nanocrystals. |
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