Functional Assembly And Property Exploration Of Semiconductor-based Nanostructure Materials

In this dissertation,various chemical methods are explored to realize the functional-assembly of semiconductor-based nanostructures and modulate novel properties,accompanied by understanding the structural and functional correlations of separate constituents.A series of multi-functional nanosructure unit are successfully synthesized,and their physical or chemical properties are systematically studied,such as magnetic,optical properties and photocatalytic activities.Further study shows that the chemical,physical or mechanical properties of theses chemically distinct heterostructures have been largely enhanced or modified,or even resulting in novel functions.1.1-D CdS-Ni semiconductor-magnetic functionally-assembled nanocompositesWidely-used semiconductors were chosen as object,1D CdS-Ni semiconductor-magnetic nanocomposites was achieved by means of electroless plating of nickel nanoparticles on preformed CdS nanowires.The growth of Ni nanoparticles onto CdS nanowires may be attributed to a nonepitaxial process.Conversion phenomena have been detected and verified in our experiments,during which the substitution rate of Cd²⁺ by Ni²⁺ on the surface of CdS nanowire is very fast,generating a layer of NiS shell with a thickness of about 5 nm.A kinetically driven conversion growth is proposed to elucidate the formation of NiS on the nanometre scale.In a parallel experiment,in the absence of hydrazine and NaOH,we obtained only CdS@NiS core/shell 1D nanostructures.In our experiment, EG as typical polyol played a crucial role.It can also bind to both Cd²⁺ ions in the shells and Ni²⁺ ions in solution,forming intermediate complexes to favour the forward reaction.It is worth pointing out that the substitution rate of Cd²⁺ by Ni²⁺ is very fast after heating.We found that only conversion of CdS to NiS on the surface of nanowires was observed for the sample(with,or without hydrazine and NaOH) processed for 10 min.With longer reaction times,the NiS layer became a little thicker and gradually reached a constant level.After that,the thickness of the NiS shell did not increase significantly even after 24 h.This suggests that once a surface layer of CdS has been substituted by NiS,it caused a passivation against further Ni²⁺ attack. The anion sublattice can be partially disrupted owning to the replacement of Cd²⁺ ions by Ni²⁺ ions.Thus,structural defects appeared in the crystal structures acting as nucleation sites for the redox reaction.The NiS shell can act as nucleation sites for the redox reaction and the nonepitaxial process of Ni nanoparticles.With the reaction proceeding,an aggregation-driven shapetransformation process may be involved to increase the size of the nanocrystal on the nanorod.Some nanoparticles could also develop into loops around the nanowires owing to the structural nature of the hexagonal-faced CdS.The six surfaces around the CdS nanowires have equal chemical activity.The M-H curves of 1-D CdS-Ni nanocomposites showed the ferromagnetic property of Ni are conserved.Compared with the neat Ni samples and bulk materials, nanocomposites showed decreased Ms and enhanced Hc value.The incorporation of magnetic Ni into the semiconductor makes it possible that the CdS nanowires can be aligned exactly at the intended positions or in a certain direction and recycled conveniently using a magnetic field,exhibiting special significance in practical applications.2.Epitaxial,nonepitaxial process to synthesis 1-D CdS/α-Fe₂O₃,CdS/Fe₃O₄ hetero-,functional-assembly nanomaterials1-D CdS/α-Fe₂O₃ semiconductor heterostructures were prepared by decorating quasicubicα-Fe₂O₃ nanoparticles onto CdS nanowires through a simple solvothermal method using Fe(NO₃)₃·9H₂O as the source materials.Both components are single-crystalline with distinguished and coherent interfaces,indicating the formation of chemical bonds between them.The structural homogeneity and the possibility of preferential crystal growth on multiple nucleation sites around the c axis of CdS nanowire have been illuminated by the simulated ED patterns and structural model of both constituents,providing insights into heterostructure formation in large lattice mismatched systems and how different geometries may be synthesized,More interestingly,We detect that even someα-Fe₂O₃ nanoparticles grown on the same facets of CdS nearly have identical shape and orientation.The optical properties and photocatalytic activities of the semiconductor nanocomposites towards organic pollutants under visible light(λ>420 nm) were investigated.Upon light absorption, difference in the positions of conduction bands drives photoelectrons generated in CdS nanowires to surroundingα-Fe₂O₃ nanoparticles,resulting in the enhanced photocatalytic activities.1-D CdS/Fe₃O₄ semiconductor-magnetic functionally-assembled nanocomposites were prepared by decorating Fe₃O₄ microspheres onto CdS nanowires through a simple solvothermal method using FeCl₃·6H₂O as the source materials.The whole Fe₃O₄ microspheres exhibited a remarkable polycrystalline feature,resulted from assembly of small nanoparticles.The growth of Fe₃O₄ microspheres onto CdS nanowires may be also attributed to a nonepitaxial process.The presence of S^2- is critical to nanocomposites formation:it can bond Fe³⁺(or Fe²⁺ after heating up) strongly and appear to aid in the attaching ofα-Fe₂O₃ and Fe₃O₄ nanoparticles on multiple nucleation sites.The optical,magnetic properties and photocatalytic activities were separately investigated.The PL behavior of CdS and ferromagnetic property of Fe₃O₄ are conserved,and the photocatalytic activities were not weakened distinctly.In summary,these types of nanocomposites are likely to find potential applications in nanoscience owing to the combination of the ferromagnetic or catalytic domain with 1D semiconductor nanostructure into hybrid nanocomposites.3.Synthesis and Photocatalytic activity of 1-D CdS nanostructure-based core/shell semiconductor composites1D CdS@ZnS core-shell nanocomposites were successfully synthesized by coating a layer of ZnS shell via a solvothermal method,using zinc acetate and thiourea as the source materials.Our results indicate that the ZnS shell with rough surface is made up of ZnS nanoparticles of～4 nm diameter growing along the surface of the CdS nanowires,while the thickness of the shell layer was calculated to be 5～10 nm.The surface modification of a wide band gap ZnS shell around a narrow band gap CdS core can passivate nonradiative recombination sites leading to high optical properties and enhanced photocatalytic activity to MB and 4CP solutions. Over the past few years,both shape and compositional control have been applied to CdS-included nanostructures,attempting to improve its stability or functional properties.However,few of the above results have been proven successful regarding both the stability and the efficiency of CdS.1-D CdS@TiO₂ nanocables were synthesized by slow,uniform hydrolysis of TBT under the presence of CdS nanowires. The experimental results showed that appropriate TiO₂ shell can protect the CdS core from contamination and promote its stability,while the presence of the interface can promote charge separation to favor high efficiency in the photocatalytic reaction, exhibiting important significance in practical applications.4.Synthesis and performance of novel Bi-Ca-O system photocatalystsA series of Bi-based photocatalysts were prepared by conventional polymer-coordination and co-precipatation methods.The details of the reaction processes were investigated and the different intermediate products were identified by X-ray diffraction after the pure phases were obtained.Moreover,the optical behavior and photocatalytic activities under visible light were investigated.