Controlled Synthesis And Characterization Of Nanostructured Indium Oxide

The controlled synthesis of indium oxide hierarchical nanostructures through liquid-phase chemical route is investigated in this thesis.The controlled synthesis,formation mechanism,and properties are investigated in detail.The thesis mainly focuses on the preparation,formation mechanism and photoluminescence properties of hierarchical nanostructured indium oxide obtained by simple solution method,which mainly includes the lotus-root-like In₂O₃ nanostructure from the aqueous solution route,the flower-like In₂O₃ nanostructure from a novel solvothermal indium precursor,and the In₂O₃/PEG400 hollow sphere from the solvothermal synthesis.This aims to study the intrinsic formation mechanism of hierarchical nanostructures in solution route and find the more effective strategy to fabricate novel nanostructures.1.Lotus-Root-Like In₂O₃ Nanostructures:Fabrication,Characterization,and Photoluminescence PropertiesNovel lotus-root-like In₂O₃ nanostructures with a diameter of ca.300 nm and a length of 1.5-4.0μm have been prepared by annealing In(OH)₃ nanostructures with the same morphology derived from a mild solution reaction.The hierarchical nanostructures are composed of several segments aggregated orderly from In₂O₃ nanorods with the length of 50-90 nm and diameter of 15-40 nm.The segments of the lotus-root-like In(OH)₃ nanostructures are composed of nanoparticles with the size of ca.10 nm and a small misorientation exists at the interface although the planar fringes are ordered in the particles,which implies an oriented aggregation growth mechanism.To track the fabrication process of lotus-root-like In(OH)₃ nanostructures, a detailed time course was studied.On the basis of the experimental results, the formation process of In(OH)₃ nanostructures was proposed.In the initial stages,when the solution was heated,the In³⁺ cations hydrolyzed to form the colloid particles due to the decomposition of formide at high temperature,and the as-formed amorphous nanoparticles subsequently aggregated to small elliptical particles to minimize their surface energy,which can be revealed by the HRTEM observation.At the same time,the transformation from the amorphous particles to cubic In(OH)₃ occurred.With the reaction proceeding, grain growth was carried out with the consumption of the reactants in the solution,the particles became larger and larger,and the spindles with the size of 500 nm were formed after the reaction was conducted for 30 min.At this stage,the grain growth exhibited anisotropy due to the anisotropy of the In(OH)₃ crystallographic structure.When the reactants were consumed completely,the grain growth almost stopped.Then the oriented aggregation along[110]direction started due to the large surface energy of(110) planes. As a result,the lotus root-like In(OH)₃ nanostructures were formed,which can be transformed to In₂O₃ nanostructures by calcination without changing the morphology of the nanostructures.The PL spectrum of In₂O₃ nanostructures at room temperature exhibits three peaks centered at 468,551,632 nm and a shoulder at 445 nm in the visible light region.These emissions may be related to the defect produced during the preparation process.2.Flower-like In₂O₃ Nanostructures Derived from Novel Precursor:Synthesis, Characterization and Formation MechanismThree-dimensional flower-like In precursor nanostructures were fabricated by glycerol-mediated solvothermal reaction using InCl₃·4H₂O and formide as reagents.The precursor composed of nanosheet and the corresponding XRD, FT-IR,TG is similar to the reported Co,Mn-based glycerol.And the as-formed indium precursor could be transformed to cubic In₂O₃ maintaining its original flower-like morphology after calcination.HR-TEM image of a nanoplate at the edge of the In₂O₃ flower-like nanostructure shows the continuous lattice fringes in the visible range,indicating its single crystalline nature with the dominated surface of {210}.To track the formation process of the precursor,TEM,FE-SEM,XRD and elemental analysis techniques were applied to investigate the samples collected at different reaction times.At first,the In(OH)₃ nanoparticles were formed due to the decomposition of formide during the heat-treatment,which aggregated to large spheres due to the high surface energy of the nanoparticles. The decomposition of formide was slow due to the small amount of water in the system,which further led to a slow formation rate of In(OH)₃ as well as the following aggregation.Then,the In(OH)₃ reacted with glycerol by replacing the hydroxyls in In(OH)₃ to form the nanoplates.However,this reaction could not carry out thoroughly because the presence of water in the system and a relatively low temperature.Finally,the crystalline flower-like In-glycerol complex precursor was formed,which can easily transforms to In₂O₃ without changing the morphology during calcination.The room temperature PL spectrum of flower-like In₂O₃ nanostructure exhibits a strong emission centered at 442 nm with two shoulders at 468 and 524 nm,as well as a weak peak at 627 nm in the range of visible light region. The emission at 442 nm and the shoulders can be attributed to the radioactive recombination of a photoexcited hole with an electron occupying the oxygen vacancies,while the emission at 627 nm may result from the Raman scattering.3.PEG-Assisted Synthesis of Nanosized In₂O₃ Hollow Structures and Their Optical PropertiesThe nanosized In₂O₃/PEG400 composite hollow spheres(70-100 nm in diameter) with mesoporous shells of 10-20 nm were synthesized by a poly(ethylene glycol)(PEG)-assisted solvothermal method using In(NO₃)₃·4H₂O and urea as reactants.The HR-TEM image shows that the hollow spheres aggregated orderly from In₂O₃ nanocrystals with the diameter of 7 nm.The continuous lattice fringes confirm the oriented-aggregation of the nanocrystals,although the interface between the particles can be clearly observed.The N₂ adsorption-desorption isotherms of the hollow spheres exhibit the typeâ…£adsorption isotherm and a hysteresis loop in the relative pressure range of 0.4-1.0,indicating the presence of the inhomogeneous mesopores, which are formed through the aggregation of the nanocrystals.The corresponding pore size distribution curve calculated from the desorption branch by the BJH method displays a pore size distribution from 3 to 10 nm, centered at ca.3.4 nm,which is close to the result from the TEM images.The calculated pore volume is 0.19 cm³/g,and the specific surface area is 94.1 m²/g by the BET method.The UV-visible absorption spectrum of this novel In₂O₃/PEG400 nanosized hollow spheres shows absorptions at 310 nm.The absorption shows obvious blue shift compared to the absorption at 330 nm of bulk In₂O₃,which arises from the weak quantum confinement effect.The room temperature PL spectrum of In₂O₃/PEG400 nanosized hollow spheres exhibits emissions centered at 461,538 and 620 nm.It is speculated that the novel emissions are related to the defect produced during the preparation process and also relate to the hollow organic/inorganic composite nanostructure.