Shape-controlled Synthesis And Catalytic Property Studies On Semimetallic Bismuth Mciro/nano-materials

Fabrication of micro/nanostructures made of various metals has beenintensively studied in the last couple of decades due to their unique properties（compared with bulk materials）, which enables them to be promising in applicationsover very broad areas including catalysis, photography, photonics, electronics,optoelectronics, information storage, sensor, biological labeling, imaging, medicaltreatment, and surface-enhanced spectroscopies. The properties of metalmicro/nanostructures are strongly dependent on their dimension, composition,crystallinity, shape and construction geometry （e.g., core-shell, solid, and hollow）.The shape-controlled synthesis has received considerable attention recently becausevarying the shape of micro/nanostructures allows one to fine-tune their propertiesover a wide range. Bismuth （Bi）, as a semimetal with a very small band overlap,provides a very attractive model system for studying physical phenomena owing tohighly anisotropic fermi surface, low carrier densities, small effective mass andmean free path. Theoretical studies predicted that micro/nanostructured Bi waspotentially useful for mesoscopic physics and fabrication of nanoscale devices,which have stimulated great efforts to synthesize Bi micro/nanomaterials.In this thesis a newly rational low-temperature solution-synthetic route waspresented for the synthesis of metallic bismuth （Bi）. Results of the thesisdemonstrated that the shape and size of micro/nanoscale Bi could be controlled bychanging the synthetic conditions, such as reaction temperature, solvent,concentration of reductive and kind of surfactants. An aqueous chemical reductionmethod is proposed to prepare Bi micro/nano crystals using different reducing agentin different medium （alkaline medium or acid medium）. We also studies the effect ofmorphologies and structures of Bi samples on their catalytic activity to degradationof organic pollutants under visible-light or without light. The structures, sizes,shapes, optical and catalytic properties of the as-prepared Bi products were analyzedand characterized systemically by using XRD, SEM, TEM, HRTEM and UV-visiblespectroscopy. The major results of the thesis are outlined as follows:Firstly, Bi with tunable morphologies have been successfully prepared on alarge scale via a simple and rapid solution phase method at90℃with hydrazinehydrate （N₂H₄·H₂O） as reducting agent in alkaline medium. We selectively prepared different shaped1D rodlike nanostructures and3D architectures of Bi by changingthe variety of PEG （such as PEG20000, PEG6000or PEG400）. These3Darchitectures were self-assembled by1D nanorods or nanoplates. The micro-bundlestructures had a length of4–5μm, a diameter of0.5–1μm, and the individualnanorods composing the bundles are10–50nm in diameter. A possible formationmechanism for the interesting architectures was proposed to interpret the growthprocess. By rationally adjusting the synthetic parameters such as the variety ofsurfactant, the quantity of surfactant, reductant concentration and reactiontemperature, Bi micro/nanostructures with bundle-like, shuttle-like, quasi-spherical,plate-like and rod-like could be selectively synthesized.Secondly, the dissertation demonstrated a work for selective preparation ofdifferent shaped Bi nanostructures via an aqueous solution process by reducingbismuth nitrate with N₂H₄·H₂O in alkaline medium. Here, Bi nanospheres andnanorods were prepared with different organic molecules （citric acid or ethyleneglycol） as controlling agent. Bi nanotubes could also be obtained by introducingdifferent kinds of controlling agents to the reaction system. By changing thesynthetic conditions，Bi nanospheres with average diameter of100nm, Bi nanorodswith length of50nm or200nm, and Bi nanotubes with length of100–200nm havebeen successfully synthesized. The dissertation also proposed the possible growthmechanism of the Bi samples by studying the influences of different experimentalconditions. And the optical properties of the samples were studied by UV-visspectroscopy. The absorption peaks in UV-visible spectra shift or broaden due to thedecreased sizes or the changed shapes of the synthesized Bi nanostructures. Inaddition, we also demonstrated a work on the preparation of Bi nanocubes and mcirospheres by reducing bismuth chloride with N₂H₄·H₂O through introducing amountsof tartaric acid （TA） to the reaction system in alkaline medium. On controlling thesize and dispersibility of the project powders, Bi nanocubes with an edge length of150–200nm, and Bi mciro spheres with size of10μm have been successfullysynthesized.Thirdly, a simple and mild aqueous solution reduction method was successfullyused to synthesize different shaped Bi nanostructures at90℃by reducing bismuthnitrate pentahydrate with sodium hypophosphite （NaH₂PO₂·H₂O） in alkaline medium.The achieved Bi nanostructures exhibited nanoparticles （10–50nm in size） orbelt-like （lengths of up to10μm and widths of up to100nm） shapes. In theexperiments, TA acted as complexing reagent, and the morphologies of the Bi nanostructures can be controlled by adjusting the quantity of NaOH used in thereaction. The quantity of NaOH determined the reduction rates of [Bi₂（C₄H₂O₆）₂]^2-,which influenced the shape of Bi samples. Low reduction rates resulted in1D Binanostructures as nanobelts, which had tendency to roll to tube-like structures.Interestingly, we found that the belt-like and tube-like structures were constructedby aligned nanoparticles of several nanometers. Results indicated that tartaric acidand NaOH are key factors in our preparation. These Bi nanostructures are expectedto find potential applications in a variety of areas due to their optical characteristics.To the best of our knowledge, this work is the first concerning on the synthesis ofmetal Bi nanostructures in aqueous solution with NaH₂PO₂·H₂O as reductant. Webelieve that the rational low-temperature synthetic route is universal and can beadapted for the preparation of numerous metal materials in solution. For example, itmight be possible to prepare Au, Ag, Cu, and Pt nanostructures via methods similarto those described in this work.Fourthly, the thesis also reported a low temperature solution reduction methodemployed in the synthesis of large quantities of micro/nano-sized Bi crystals withNaH₂PO₂·H₂O as reductant in acidic solutions. The achieved Bi crystals exhibitedplate-like （100nm in size and few nanometers in thickness） or polyhedral （500nm insize） shapes. The experimental results suggest that the polyhedral Bi samples weresynthesized by prolonging the reaction time. The assembly and oriented growthshould be the reason. Optical properties of the Bi samples with different shapes werealso investigated by UV–vis method. In addition, it was found that the introductionof TA to the reaction system was essential for the preparation of1D bismuthnanostructures with NaH₂PO₂·H₂O as reducing agent in acidic solutions. Theachieved1D nanostructures have typical diameters of30–50nm and length of up to5μm. The effects of reaction parameters in the system on the preparation of Binanostructures were studied. TA was used as the shape controlling agent, which wasa key factor for getting1D structures in the preparation. Bi nanorods andnanobelt/tube-like structures can be selectively prepared by changing the reactiontemperature.Finally, based on the Bi samples with different structures obtained above, westudies the effect of morphologies and structures of Bi samples on their catalyticactivity to degradation of organic pollutants under visible-light or without light. Thecatalytic activity of the Bi samples was discussed via UV-vis results. Resultsindicated that the Bi samples exhibited highly activing for degradation of organic pollutants.3D hierarchical bundlelike Bi structures （the degradation rate of Rh B canreach up to99%after70min’s under visible-light irradiation） had betterperformance than the dispersed1D Bi nanostructures （the degradation rate of Rh Bcan reach up to99%after150min’s under visible-light irradiation）. The Rh Bsolution can be thoroughly degraded after70min with the sample of bundle-like Bistructures under visible light irradiation. Bi with the same morphology performeddifferent catalytic activities in Rh B solutions with different pH value. It was foundthat the degradation efficiency increased evidently when the pH value decreased. Inaddition, we also discussed the degradation of Rh B with the Bi nanostructures ascatalysts under various size.100%,98%,97%and68.6%degradation of Rh B isobserved after90min visible light irradiation for Bi nanoparticles （20–50nm）,nanocubes （150–200nm）, sub-micropolyhedra （400–500nm） and micro-spheres （10μm）, respectively. The results indicated that if the grain sizes of the Bi samples arereduced from micro-size to nano-size, their catalytic activities dramatically increase.Furthermore, the Bi samples could be easily recycled six times with little decrease ofthe catalytic activity under visible light irradiation. Bi samples exhibited differentcatalytic activities to Rh B solutions under different illumination. Bimciro/nano-structures under visible light irradiation showed higher catalytic activitythan that in the dark.In conclusion, Bi micro/nanomaterials with different dimensions are obtainedby aqueous synthesis method. Our work gives an improved progress on the mildsolution synthesis of Bi micro/nanomaterials. The obtained Bi micro/nanomaterialsare promising in optical and catalytic application.